Not revealed until late in the series, Hiei is a forbidden child, born to a race of Ice Maidens that only give birth to a single girl unless a member had violated the laws of the society, and had sex with an outsider. Boys born to Ice Maidens had always had cruel natures, and would often turn violent even at an early age. Hiei himself is surrounded by a fiery aura even after birth, and can't be held unless he is first wrapped in power-sealing talismans. In fear of their own safety, the tribe of Maidens forces a woman, Rui, to throw Hiei off the edge of their floating island. Before she does, however, Rui gives Hiei a Hirui stone, a precious jewel formed from the tears of Ice Maidens. This one is particularly special, as it is one of the two Hina, Hiei's mother, shed as she had given birth to twins, and is a different shade than normal stones. Rui also hopes that Hiei would one day return to the land of the Ice Maidens, and kill them all, starting with Rui herself.

Despite being an infant, Hiei is aware of everything that had happened, and is more than willing to do just as Rui wanted. He somehow survives the fall, and is found by a group of bandits. The leader tries to take the Hirui stone from the baby, but Hiei refuses to let it go, and bites the finger of the man who would become the closest thing to a father Hiei ever has. As he grows up, Hiei realizes that he likes to fight, and to kill. Once he learns the Hirui stone is precious, he begins wearing it openly to purposely provoke people into attacking him. He enjoys the lifestyle, even forgetting about the Land of Glaciers where the Ice Maidens lived for a time. Hiei is an incredibly powerful demon, however, and eventually is shunned by the bandits who took him in and now fear him. Hiei moves on, traveling to new lands, and continues to fight, using his Hirui stone as temptation. It costs him, however, as one fighter cuts the cord Hiei uses as a necklace. While Hiei is able to kill the fighter, he loses his stone in a river, not realizing until then how important the item is to him.

Eventually, Hiei makes his way to a demon named Shigure, who can implant Jagan eyes into demons. The Jagan is a powerful thing, and among other abilities, allows the user to find things that they are seeking. Shigure at first refuses, declaring Hiei too young and weak to withstand the substantial pain involved. To prove his tolerance and his willingness, Hiei runs his hand through his katana, and demands to have the operation. Shigure, however, is a particular demon, and says he will only perform the transplant if Hiei has lead a life Shigure feels drawn to. Hiei's tells his story, which makes him sick to even think about, and earns the transplant. The operation is successful, but puts Hiei through pain like he has never felt before, or since (including through all of the events in this history). It also drains a great deal of Hiei's power, lowering his level from the elite A-Class category to that of one of the lowest ranks assigned by the Spirit Realm, the D-Class. Because he is so weak, Shigure insists Hiei learns true sword fighting before letting him leave.

Within days of leaving Shigure, Hiei uses his Jagan to find the Land of Glaciers. He doesn't kill any of them, however, as he moves through their village, and sees how lifeless and full of fear they all are. Their state simply makes him lose his desire for revenge. Eventually, Hiei finds Rui, and when asked to see Hina, is taken to her grave. Hina committed suicide shortly after Hiei was cast off the island. Rui also tells Hiei about his younger twin sister who has left the Land of Glaciers after hearing about her brother's fate. Rui recognizes Hiei and begs him to kill her, but he does not.

After he leaves the Land of Glaciers, Hiei decides to look not only for his still-lost Hirui stone, but for his sister as well. He returns to Shigure (though the reason why isn't made clear), and tells the man of what he learned. Shigure decides that per his habit of taking part of his patient's lives as his fee, he will take Hiei's promise to never tell his sister that he's her brother as payment for the transplant. Hiei claims he never intended to tell her anyway, and heads to the Human Realm, where he senses the girl. He does find her once, however he can't bring himself to talk to her. Eventually Hiei meets up with another demon that is hiding in among the humans to restore his strength named Kurama.

The series begins a few years later, and Hiei first appears as one of the villains, working with fellow demons Kurama and Goki to steal the Three Great Artifacts of Darkness from the Spirit Realm vault. Hiei's chosen item is the Koma no Ken (Blade of Demonic Subjugation), and has the power to turn any human it cuts into a weak, stupid demon. Until this point, nothing is known about Hiei in the Spirit Realm. Despite this, and the sheer lack of experience of his new Spirit Detective Yusuke Urameshi, Koenma is forced to send the teen after the demons, only having a week to get the items back without his father potentially destroying the world to reclaim them.

When Yusuke first comes across the trio, Kurama is attempting to withdraw from the alliance. Hiei is clearly displeased with this, and chases after Kurama, leaving Goki to face Yusuke alone. With his speed, it's doubtless that Hiei caught up to the teen. However, he doesn't take the item, likely because he doesn't want to get into a conflict with the fox demon.

Goki is ultimately defeated, and Kurama willingly surrenders his item after he uses the mirror to save his mother's life. Hiei uses the power of his Jagan eye to watch all of this, and resolves to kill Yusuke without making the same mistakes as the other two. After finding out about Yusuke, Hiei kidnaps Keiko Yukimura, who the teen loves. He telepathically relays through Yusuke's assistant Botan that Yusuke must bring the two artifacts he has won, or Keiko will die. He manipulates his Yoki, or demon energy, to lead them to a warehouse, where he has control over multiple humans through the use of his Jagan. Once Yusuke hands over the items, he returns Keiko to her, but not unharmed: Hiei has cut the girl with the Koma no Ken, ensuring that she will turn into a demon unless Yusuke can beat Hiei, and get the antidote in the hilt of the sword. Hiei has a bit of a flaw then in that he tends to talk too much, letting Yusuke get the upper hand despite Hiei's incredible speed. In order to defeat Yusuke, Hiei transforms into a rather ugly creature, with green skin and Jagan amplifiers that open all over his body. He nearly beats Yusuke, about to run him through with the Koma no Ken until Kurama shows up, and takes the blow instead. With a smirk, Kurama cuts his own hand on the sword, and uses his blood to blind Hiei's primary Jagan, the source of his power. This buys enough time for Yusuke to recover, and in a desperate gamble, he fires his Rei Gun at the mirror Kurama used, reflecting the blast off of it to hit Hiei from behind since the demon was too fast to hit dead on. It works, and Hiei passes out.

Several months later, Yusuke has just gotten back from training with Genkai in time to get another case, this time needing to go to Labyrinth Castle to stop four demons called the Holy Beasts from infecting the world with demonic insects that make humans go homicidally insane. Kuwabara, Yusuke's best friend, goes with him to the castle, but Koenma decides to offer Kurama and Hiei a chance to avoid imprisonment by letting them help Yusuke. Despite this, Hiei proudly proclaims he is there to steal whatever treasures he can find in the castle, not to give Yusuke a hand. Kuwabara gets a bit sassy with the demon about his coldness, and Hiei threatens his life. When Kuwabara tries to attack Hiei, however, Hiei steps forward, completely ignoring Kuwabara to promise Yusuke that he will take revenge on the detective before stepping back beside Kurama, dodging another punch from Kuwabara.

The four head into the castle, and are immediately tested by the Gate of Betrayal, so named because it puts just enough weight on its victims to force them to use all their might to keep from being crushed, but leaving enough leeway for a single person to escape, ensuring the death of the ones left behind. If no one goes, the group will eventually tire, and everyone will die anyway. While Kuwabara thinks it would be noble to die as a team, Hiei mocks the idea, and threatens to leave, but is stopped by Kurama's declaration that he better not. Yusuke then tells Hiei that since he's the fastest, he should run to the lever that activates the gate and shut it off. Yusuke plans to use his spirit energy to support Hiei's portion during this time. Kuwabara is convinced Hiei will betray him, and Hiei says he just might, since he's already promised to take revenge on Yusuke. Yusuke remains confident, however, stating that Hiei is the kind who will take his revenge directly, and that he trusts the demon.

Hiei runs to the lever, and reaches out to pull it, but hesitates. The Gatekeeper tempts him, telling him not to be so rash, and that there is a place for him in the Holy Beasts' army. Hiei pretends to go along, even laughing at the others' suffering, until the Gatekeeper turns away. At that point he strikes, slicing into the thing with a non-fatal blow, and ordering it to tell its masters that they should beg for mercy. Hiei then turns off the Gate, freeing the others. When Yusuke goes to thank him, however, Hiei frankly says that the only reason he did it is because the greater numbers make his own efforts easier, not because he likes them. Kurama interprets this for Yusuke and Kuwabara as Hiei's "you're welcome," and the four move on.

The first Beast they encounter is Genbu, who Kurama decides to fight. Hiei reveals that he originally teamed with Kurama so he wouldn't end up fighting against him when Yusuke and Kuwabara question the teen's fighting prowess, saying that Kurama is even more heartless in battle than he is. Kurama ultimately wins, but is injured, which surprises Hiei.

Kuwabara claims the next fight, and shows off his Rei-ken, prompting Hiei to claim he's not entirely useless. Kuwabara takes the next fight against Byakko, and has a hard time at it, first against Byakko's enchanted beats, and then against the demon himself, who has the ability to devour Kuwabara's Reiki, or spirit energy. The battle moves from outside a tower to a cave with a floor covered in lava that's hot enough to dissolve anything that falls in. Kuwabara nearly dies in order to defeat Byakko, punching him off his platform into the lava with a suicidal punch that's guaranteed to send Kuwabara to his grave as well…except that one of the bandages Kuwabara wraps around his torso comes loose, and catches onto the platform's jagged edge. The group takes the time to tease Kuwabara before they move on, with Hiei saying that since he cheated death, they should let Kuwabara fight again.

The next room they come to has an ostentatious door, as described by Hiei, decorated with fierce-looking statues of dragons. It's the room of Seiryu, an ice user. Before the battle can begin, however, Byakko returns, having survived his battle, but barely. Byakko begs Seiryu for a bit of energy so he can live, promising to return it as soon as he can, but Seiryu refuses, and kills Byakko by freezing him, then shattering his body with a punch. Yusuke starts to step forward in his anger, but Hiei tells him to hold his rage for Suzaku, the last of the Beasts. Hiei had originally intended to fight Suzaku himself, but decides to take this battle instead, and removes his cloak to cover Byakko's decapitated head. As he prepares to fight, Kurama notes that Hiei's actions are contradictory to his nature; Hiei would have likely done the same as Seiryu not long ago. Kurama notices Hiei's confusion over it, but also points out that the bloodlust Hiei is beginning the battle with is stronger than any he's ever seen Hiei use.

Seiryu offers Hiei a final chance to beg for forgiveness, but Hiei will have none of it, and smiles as he promises Seiryu death. The battle begins with Hiei using his speed to dodge Seiryu's freezing punches, but the method only works for so long, as Seiryu eventually freezes Hiei's leg. Forced to make an attack, Hiei uses his sword to launch himself at Seiryu, and ends up getting his other leg frozen, along with an arm. It doesn't matter, however, as Hiei is easily able to free his limbs, and has struck Seiryu with sixteen blows that kill him. Hiei is able to move faster than anyone else, his strikes rapid enough for Yusuke to only count 15, Kurama to count 1 followed by flashes of light, and for Kuwabara to miss them entirely. Kurama again notes Hiei's changing behavior, and credits it to Yusuke's influence.

While approaching the final tower, the group is halted by a mass of mindless zombies that feel no pain or fear. There's little time to fight them, so at Hiei's suggestion, the group forms a human ladder that allows Yusuke to leap to a window and continue on without the others. By the time the other three do catch up, Yusuke has defeated Suzaku, but at the cost of his own life energy, the desperate measure required to save Keiko in the human world. Kuwabara transfers some of his Reiki into Yusuke to save him, an action that Hiei doesn't understand. To him, it makes no sense to wear yourself out for the sake of someone else, and despite the fact that it's how they won, Hiei claims he could never do it for anyone else. Hiei and Kurama take Yusuke and Kuwabara back to the human realm, and all is well.

At least for a little while. Shortly after their return, however, Hiei is sent by Spirit Realm to deliver a tape to Yusuke with the details of his next case. It's strange for a few reasons, especially since Hiei hates taking orders, but Hiei makes the delivery and goes. Yusuke, Kuwabara, and Botan watch the tape at Kuwabara's house, and learn that there is a young youkai woman named Yukina being held prisoner by crime lord Gonzo Tarukane for the special jewels made from her tears called Hirui stones. When he sees the image of Yukina on the video, Kuwabara declares that he is in love with the girl, and rushes off before watching the rest of the video, which reveals that the mission is especially important because Yukina is Hiei's younger sister. It's revealed that Kurama had the idea of having Hiei deliver the tapes in hopes that Hiei would trust Yusuke with the rescue, as it is probable that Hiei would kill every human involved in the organization for harming the girl; youkai killing humans is strictly forbidden by Spirit Realm law, no matter what the reason. There's also hope that maybe Hiei doesn't know what the mission is about at all.

Of course, Hiei being Hiei, he not only knows, but also trails Yusuke, determined to rescue Yukina himself.

Yusuke and Kuwabara go through several fights in order to save Yukina, eventually defeating a pair of demons with incredible youkai known as the Toguro brothers. While they are fighting, Hiei breaks into the mansion where Yukina is being held, making his way to the observation room where Tarukane has been watching the fight. Knocking out the remaining humans in the room, Hiei begins to pummel Tarukane in front of Yukina. He is ready to land the punch to kill the crime lord, but Yukina stops him, begging Hiei not to force her to see more hate and suffering. Hiei lets Tarukane go, saying the human isn't worth Yukina's tears, and claiming only to be a friend of Yusuke's.

The rescue complete, all involved return to their lives, including Yukina. Kuwabara, Yusuke, and Hiei all watch as she returns to her home, with Hiei reasserting to Yusuke that he has no intention of ever telling Yukina that he's her brother. Peace never lasts for long, however, and soon Yusuke and the others learn that the defeat of Toguro was just a ruse; Toguro's power is much, much stronger than any member of Yusuke's team. In order to make Yusuke stronger, Toguro invites him and the others to an event called the Dark Tournament. All must attend, else they and their families will be killed. They must also find a fifth man for their team, as per the tournament's rules.

While Yusuke goes to Genkai to train for the two months, Hiei and Kurama take on the task of training Kuwabara. Hiei's part of the training comes first, and largely involves beating Kuwabara with a bamboo shaft to make him tougher. It's not until his training with Kurama begins that Kuwabara is told Hiei purposely held back because of Kuwabara's weakness as a human. We don't see the details of the training other than these brief scenes.

The tournament is to take place on a remote island, and to get there, numerous teams will be transported by boat. All of the teams except Yusuke's that are being transported are made up of youkai. Hiei, Kurama, and Kuwabara are waiting on Yusuke to arrive with their fifth member. Yusuke and the fighter, who is called "Mask" for the mask she wears, almost arrive too late to board. In greeting, Hiei immediately attacks Yusuke with his sword to test his strength, and is impressed as Yusuke dodges each blow with ease before finally grabbing the blade between two fingers to stop it.

Once on board and headed to Kubikukuri Island, the captain of the ship announces that only one of the many teams will actually compete at the tournament, as the other 15 have already been chosen. The competition is a battle royal, which one member from each team fighting to see who will advance. Because Yusuke is sleeping, Mask steps forward to battle, which is fine with Hiei, who wants to see her strength. When Kuwabara expresses concern that she might lose, Hiei calmly states that if she does, they'll only need to kill everyone else on board, and say they are the winners. Then no one can complain. Mask does win the fight, but the other youkai decide to use Hiei's idea for their own. Eventually, everyone except the captain and Yusuke's team is killed, and they arrive.

The group settles into the hotel room for the night, and is brought five cups of coffee. Kuwabara, thinking his might be poisoned, refuses to drink his, and Yusuke is still asleep, meaning that there should be two coffee cups left. Kurama, however, notices that only one cup is unclaimed, and a slurping noise from the wall alerts the group to Rinku's presence. He's a member of team Rokuyukai, who Yusuke's team will fight first, and there to intimidate them a bit. Also present is a youkai named Zeru, who tries (and fails) to wake the sleeping Yusuke by using his yoki to slice through the remaining coffee cup.

The tournament official begins the following day, and the two teams decide to battle one on one, with the best of five fights advancing. Kuwabara fights first against Rinku, and despite his best efforts, eventually loses…but Hiei notes that it's only because the rules allow for count outs. Otherwise, Kuwabara would have won. The second fight is Kurama versus Roto, and has a bit more at stake, as Roto has sent an errand demon after Kurama's beloved human mother. Kurama manages to plant a special seed inside of Roto, however, that eventually prevents him from signaling the errand demon to kill Kurama's mother. The plant kills Roto, and Hiei reassures Kurama that once Roto dies, his errand demon has died as well. The pair don't let the others in on the secret, however, only saying they'll disclose the details to Kuwabara and Botan "later."

Zeru steps forward for the third fight, and Hiei quickly claims the battle, as he's wanted to repay the youkai for the previous night. He points out the unrefined nature of Zeru's technique, noting the edges of the cup that appeared to have been sliced the night before were in fact sloppily melted. Zeru begins attacking with fire blasts at first, which Hiei dodges fairly easily. Deciding to power up, Zeru releases a great deal of fire only to absorb it again, which causes his body to glow a bright red. His speed increases, and he manages to run Hiei through with his hand, setting him on fire before striking him with an explosive blast. It seems that Hiei has been killed, much to the shock of everyone, and Zeru is walking towards the ring's edge when Hiei stands up, appearing unharmed.

He compliments Zeru's power as he prepares his retaliation, summoning the black flames of the Demon Realm. The headband he wears burns away to reveal an open Jagan, and Zeru cowers in fear as Hiei tells him of his approaching death:

"Your skill is fine! So much so that it would be a shame to kill you…You ran out of luck when you came up against me. Rejoice! You are the very first victim of the Jao ensatsu-ken (Jagan techniques, basically) here in the Human Realm!…The rules are that I just have to defeat you. However, right now I cannot completely control this myself. If I were able to I would have liked to let you get through this without killing you. Using my right arm alone will be plenty. Do you see the delicate difference now, between how you just play with fire, and a truly demon-laden flame technique?! Do not look down on the power of the Jagan!"

By this time, the flames are massive, and Hiei launches a powerful dragon from his right arm called the Kokuryu-ha, or Dragon of the Darkness flame. The resulting cinders on the far wall of the arena are all that remain of Zeru once the attack is finished, but Hiei is not unharmed: his right arm is badly burned from his fingers to above his elbow. Kuwabara worries that Hiei will come after them with that attack, but Hiei reassures him by saying that they are allies at least until the tournament is over, and that the ability isn't perfected yet. Kurama sees the true cost of the attack, however, knowing that Hiei would have never used the dragon had he been able to avoid it, and likely won't be able to use his right arm for some time, if ever again.

Yusuke wins the final battle against Chu, the final member of the Rokuyukai who killed the other two team members when they tried to run away, and Yusuke's team advances to the next round. The damage is done, however, as the next day Hiei can't even hold his sword in his dominant right hand, and is shown trembling in pain after trying.

On the third day, Hiei is again out trying to regain use of his right arm before the team's second match. Kurama goes to find him, and comments that it will be difficult to go on without use of his dominant hand. Before they can proceed to the arena for the match, however, two members of the Dr. Ichigaki team, who are their next opponents, stop them. The first is killed quickly, but the second summons a robot/demon hybrid to fight. The battle takes time, successfully keeping Hiei and Kurama occupied while the official match begins in the arena. Eventually, Kurama defeats the machine, and he and Hiei learn of the full extent of Dr. Ichigaki's plan, which is poisoning a human teacher so he could volunteer to cure him. In exchange, he took three of the desperate students for experiments, brainwashing them into murderers. They find the teacher and cure him, bringing him to the arena in the middle of the fight. The humans are eventually saved and the match won by Mask in a Reiki-draining attack, and not before Kuwabara is seriously injured. Dr. Ichigaki is thoroughly walloped by Yusuke, and things are wrapping up…

When it's announced that Yusuke's team will immediately begin their next fight. The rich crime lord owner of the Mashotsukai Team has bribe the members of the tournament committee, who decide how the event will be run, in order to get his team an advantage. The dirty play doesn't end there, however, as it's announced that a medical check will be preformed on Yusuke's team. Hiei and Mask are taken to a tent for a more thorough check up, but it's really a trap. The nurse is a youkai named Rukka, whose specialty is putting up seemingly unbreakable barriers. While Hiei is pissed, Kurama points out that it's really less of a handicap, as Mask's power is low, and Hiei is seriously injured. The terms of this match are that the first team to 5 wins advances, which each member eligible to fight so long as he doesn't lose a match.

Kurama takes on the first two fights and wins, but at great cost: Gama, his first opponent, seals Kurama's yoki in his body. In order to defeat Toya, the second opponent, Kurama has to sew the same plant he used on Roto inside his own body, allowing it to access the yoki and grow. He passes out as a result of the fight, but is still standing in the ring, which the corrupt committee use to allow the third fighter, Bakken, to gain an easy win. Bakken shows a sadistic streak, however, pounding mercilessly at the utterly defenseless Kurama. He's about to kill Kurama until the leader of his team yells at him to stop, because both Hiei and Yusuke are ready to damn the rules and kill everyone in the arena in order to save their friend.

Yusuke beats the crap out of Bakken, and defeats Jin with a great deal of effort. However, the committee claims that the referee was slow to start a count when Yusuke was outside of the ring, and declares the match a double loss, meaning that Yusuke is ineligible to continue. Because losing the tournament means death for not only the members of Yusuke's team but their families, Hiei suggests they turn the thing into a survival game, and kill everyone in the arena who dares to oppose them. It's revealed that somehow, Rukka's barrier is having a healing effect on both Hiei's arm and his power, doing a great deal to restore both. Before the slaughter begins, however, Kuwabara points out that while he is injured, he's still legally able to fight. He would have lost, however, had it not been for the unexpected appearance of Yukina, which somehow gives Kuwabara a power boost and allows Yusuke's team to advance.

The final matches of the preliminary rounds conclude the following day. Hiei and Kurama both show up to watch whom their next opponent will be, though the fight finishes so quickly that the others miss it. There is a bit of playful banter (including teasing Hiei about how they should help Yukina find her long lost brother) before the Toguro team appears on the other side of the arena. They stare each other down for a few moments, after which Mask points out Yusuke's weakness. The two go off to "settle things," which turns out to be a final trial for Yusuke to gain the power he needs to defeat Toguro.

It takes time, however, and both Mask and Yusuke are absent at the start of their semi-final round against team Uraotogi. The terms are the same as in the previous fight, with one exception: to determine the fighters, a die is to be rolled for each team. Five sides each have the name of one team member on it, while the sixth is a free choice. Whoever's name is rolled must fight so long as they are present and alive. Hiei adds that whenever a named is rolled for a person who is absent from Yusuke's team, he will take the fight.

The first match is between Hiei and Makintaro, and doesn't last very long. Hiei slices off Makintaro's arm so quickly, the man doesn't know it's even gone for at least a minute. Makintaro can respawn his limbs, however, and reshape them into weapons. He comes at Hiei from behind with an axe fist, but Hiei moves far too quickly, jumping onto Makintaro's back, and running his katana through the man's head.

The second match is Hiei vs. Kuromomotaro (Kuro). Before the fight begins, Kuro consults with Onji, the team leader, who reveals that Hiei is only at about 60% of his strength, and unable to use the Dragon of the Darkness flame. Kuro steps forward, removing Hiei's sword from Makintaro's body, and does a very strange thing: he purposely slices deep into his arm with the blade. It's to his own advantage, however, as he then devours the energy of a millet dumpling, and transforms his body so it is under the armor of the ape. The power of the dumplings is not only to strengthen and quicken, but to make Kuro immune to whatever attack Hiei has inflicted on him, which is proven true as Hiei's sword shatters when he attacks Kuro with it. Hiei then summons the flames of the Human Realm to use the Jao ensatsu rengoku-sho, or Fists of the Mortal Flame, which are basically fire-surrounded punches. He does a good bit of damage, but Kuro uses another dumpling and is healed, now wearing the armor of the Pheasant.

Kuro now has a clear advantage, as he's stronger and faster than Hiei as well as being immune to the attacks he can use. But to show off, Kuro uses a third dumpling, the maximum number he can use for the round, and transforms into the armor of the dog. He backs Hiei into a corner, where Hiei's foot hits the broken handle of his sword, and with a resigned sigh, Hiei picks it up. He calls the attack he's about to use "a crude technique that bears such an unpleasant image that [he'd] seriously rather not have to do it." Kuro strikes at Hiei, and is sliced into pieces by the Sword of the Darkness Flame, which bears a striking resemblance to Kuwabara's Rei-ken (the reason why Hiei doesn't like it.)

Kurama fights Uraurashima next, and Hiei notes the bad habit Kurama has of not deciding the method of attack until he notices the opponent's nature. Uraurashima traps Kurama in a periphery field and unleashes a smoke that will make Kurama revert into a baby and beyond. What beyond means for Kumara, however, is the fox demon Youko, the incredibly powerful version of himself from before he had to merge with a human in order to survive a bad injury. Hiei is pleased by the brief sight, saying Kurama is as he should be in that form. Uraurashima is killed by Shishiwakamaru (Shishi) when he threatens to betray the team by telling their secrets, and Kurama is declared the winner.

Shishi is the next fighter for team Uraotogi, while Yusuke's team gets a free choice. Everyone is eager to go, so Kurama suggests a game of Jenken to settle things fairly. Hiei, however, has no clue what he's talking about, and thinks the game is ridiculous after he's shown. After several attempts, Kuwabara finally wins, and steps into the ring. Shishi defeats him rather easily, though, sending him to an unknown place (which turns out to be the old arena), with the cloak of no return.

Mask's name is rolled next, and Hiei is about to step up when Mask appears. She's so weakened, however, Hiei thinks it's a different person entirely. Shishi does as well, and slices away her mask to reveal an old woman where a young one had been seen before during the fight with Dr. Ichigaki's team. Shishi demands that Yusuke's team be disqualified, and the crowd loudly agrees until of all people, Toguro comes out to explain that the old woman and the young woman are the same, the appearance of youth only an effect of using her technique: Mask is the famous Genkai.

Genkai beats Shishi, and after a very brief, idiotic loss by Kuwabara, takes down Onji as well. Her fights aren't over for the day, however, as she meets Toguro in battle, a man she once fought with and loved. Toguro has incredible strength, even at a restrained 80% of his power, which is sensed by everyone on Yusuke's team. Hiei and Kurama also sense when Toguro kills Genkai, which is seen by Yusuke, who is devastated by the loss.

There is another day before the finals, and Hiei is determined to master the Dragon, using the foolproof method of punishing it (and his arm) when it disobeys. When he returns to the group that night, his arm is badly burnt, worse than after he used the attack initially. It doesn't seem to pain him so much, however, as the next day he uses his arm freely at the tournament. At this point, among the team members, Kuwabara is the only one who doesn't know Genkai is dead. Team owner Sakyo steps forward to complete the Toguro team line-up, while Koenma joins Yusuke's team (with no intention of fighting, thanks to his trusty jetpack) to replace Genkai.

The first battle of the tournament is Kurama versus Karasu. Karasu has Kurama overpowered until he transforms into Youko, but the change is shorter than Kurama thought, and Karasu isn't defeated before he turns back. Karasu uses his power to make bombs explode against Kurama's body, seriously injuring him. Kurama falls, and in a desperate attempt to win and kill Karasu, he burns away what he thinks is his life energy to summon a blood sucking plant. But he has been growing stronger without realizing it, and survives the effort…but loses the fight. The referee's count had started when Kurama fell, and he neither stood nor hit Karasu in time to win, losing by .28 of a second.

Bui steps forward to take the next fight. Hiei tells Yusuke he originally planned to take on Toguro, but out of consideration for Genkai, he'd take this battle instead. The size difference is incredible, as Bui is almost twice Hiei's height (9 feet vs. 4'10). Bui begins the fight by summoning a massive axe, and attacking Hiei in multiple ways, all of which are easily dodged or blocked. Hiei actually starts to get angry at Bui for not taking him seriously, at which point Bui starts to remove his armor. Unlike the traditional use, however, these pieces are used to contain Bui's power, which is too great even for him to control. Bui is surrounded by a glowing battle aura once the armor is removed, strong enough to make him levitate, which is extremely rare. Bui begins to overpower Hiei, and while Hiei does manage to get a tracking attack to hit his opponent, Bui remains unharmed. He warns Hiei that if he doesn't use the Dragon soon, he'll kill him before he has the chance.

Hiei obliges, at last removing the bandages around his right arm that Kurama notes are not for medicinal purposes, but for sealing away his power. A "tattoo" of a dragon winding up Hiei's arm is revealed, and power begins to surge from it, black flames swirling and filling the stadium. This Dragon is much more powerful than before, Hiei having to launch two separate blasts to complete it. Bui manages to catch the monster, holding on as it takes him around the arena, killing hundreds, and demolishing most of the ring along with a fairly large chunk of the stadium wall. Bui survives, however, and manages to turn the dragon back at Hiei so it swallows him instead.

Hiei isn't dead, however, and reveals that this was the true power of the Dragon's attack, and what he wanted to show Bui all along. The Dragon is not meant to be a projectile attack, but a nutritive to the user. Hiei can now take Bui's hits like they are nothing at all without even attempting to dodge. He easily overpowers Bui, who asks Hiei to kill him now that he's lost for the second time in his life. Bui reveals that the first time was to Toguro, who saw potential in him then that Bui hadn't known was there. He trained hard, and indeed grew stronger, but Bui knows that he is now at his maximum, and thinks there's no point in living. Hiei refuses, going back into the ring to secure a win by count out. His reason is just that he doesn't like taking orders, and that if Bui really wants to die, he should do it himself.

The attack is amazing, but has one very notable drawback. After the match, Hiei has to "hibernate for several hours," as he puts it. He passes out mid-threat, warning Yusuke and Kuwabara that he'll make them pay if they lose. Nearly six hours later, he wakes up expecting the tournament to have ended, but the final matches have been delayed while a replacement ring is brought in. Kuwabara defeats Toguro the Elder, while Yusuke defeats Toguro the Younger; both battles are extremely difficult, with Yusuke having to be deceived into believing Kuwabara has been killed in order to reach his full potential.

After the tournament is decided, Sakyo, who has risked his life on Toguro the Younger's victory, activates a device that will blow up the stadium, destroying both him and his efforts to open a tunnel between the Human and Demon Realms. Everyone manages to escape unharmed, though still saddened by the death of Genkai. But the victors of the tournament receive a prize, and for Yusuke's team, their wish to have Genkai returned is granted, and Koenma brings her back to life.

Several months pass in peace, and a few of our heroes, including Hiei, grow a bit too cocky with the lack of challenging opponents. There's been relatively little activity, save for an increased number of demonic insects, but Hiei decides it has nothing to do with him. Yusuke, however, ends up being pulled into a situation by force, as three human teenagers kidnap him. The kidnappers leave a note saying that Kuwabara, Kurama, and Hiei must all come to Yojigen mansion, or Yusuke will die.

After some effort, Kurama, Kuwabara, and Botan manage to find Hiei by blowing a special whistle that only those with high spiritual awareness can hear. While Kuwabara's powers have vanished since the tournament, Kurama finds the noise terrible, and the sound is enough to make Hiei fall out of a tree. Hiei is shown the note, and smugly says that's what Yusuke gets for getting too cocky. Kurama tries to get Hiei to agree to come along by sparking his curiosity, but Hiei refuses, claiming to be unwilling to help someone who would be stupid enough to get himself caught after such a huge life or death conflict as the Dark Tournament. Botan's offer is much more tempting, as she promises to have Hiei taken off probation, giving him the freedom to leave the city should he come along. Hiei agrees, and the group of four head to the mansion.

A notice is hanging on the door when they arrive, saying that no one who enters may say the word "hot." However, it doesn't list the penalty. The group heads in and makes their way to a room heated to tropical temperatures, where a boy named Kaito Yu waits. He is a classmate of Kurama's, though Kurama has told him nothing of his demon life, nor of Hiei and the others. Kaito is a brilliant student, talented in the literary arts, though he has come second overall to Kurama's scores. Hiei tries to threaten Kaito, but Kaito's power of "taboo" prevents any sort of violent action from taking place in what is called a Territory, or the area in which gifted humans can use their special powers. The notice on the door is related to this, as the group now realizes, making "hot" a taboo word. Hiei, being Hiei (read arrogant yet lovable idiot), refuses to be intimidated by such a silly thing, and is provoked by Kaito to say the forbidden word.

Hiei's soul is then promptly sucked from his body. Good job, Hiei.

Thankfully, Kurama is quite a bit smarter than everyone else (Kuwabara and Botan both lose their souls as well), and manages to outsmart Kaito with a more complicated taboo, making him say the forbidden sound, and lose his soul. Once Kaito is incapacitated, the other three's souls are returned. Hiei is obviously embarrassed by his rashness when Kuwabara blames him for putting the others through the trouble of trying to get his soul back first.

The group moves on, now with the second kidnapper, Yanagisawa (Yana) leading the way. He takes the group to a room with 7 staircases, saying each person must take a different one up to the second floor, where Yusuke is being held. In order to keep track of everyone, Botan suggests they all write their names on special seals, and then affix one seal with each of their names to every other person's shirt. The seals read the wavelength of the person who affixes them, and can only be removed by that same person. In addition, each seal will change color to indicate the health of the affixer, and will fall off should they die. While Hiei is willing to put the seals on the others, he doesn't want to wear any, even grabbing Botan's wrist to stop her—but Kurama sticks his onto Hiei's chest, and remarks that Hiei's already lost his soul once already. Embarrassed, Hiei lets Kuwabara and Botan attach theirs.

Each member of the group seems to make it upstairs unharmed, with no changes in the color of the seals. When they arrive, the third of the kidnappers, Kido, is holding Yusuke captive by standing on his shadow; Kido's Territory allows him to completely immobilize a person in this manner. Kido reveals that one of Yusuke's friends is actually a fake, and that the group has kidnapped the real one and is holding him or her hostage. Yana has used his power of Copy to take the person's place, copying their appearance and their voice. Kido gives Yusuke ten minutes to figure out which is the fake, and to punch them as hard as he can in order to knock them out, as that is the only way to deactivate Yana's Territory. Yusuke attempts to ask various questions, including asking Hiei who his younger sister is, to which Hiei replies that Yusuke knows who she is and Hiei doesn't have to tell him. The group gets a bit sidetracked as Kuwabara demands to know who Hiei's sister is, especially once he learns that everyone else already knows. A potential crisis is averted, however, once Yusuke notices the seals, and the group realizes they have a way to find the answer.

Or not, as every person there is able to pull of the seals. Hiei believes they've been led on, but Kido reveals the full extent of Yana's power: not only does it copy the physical appearance, but also the voice, memories, personality, and spiritual wavelength of a person. He also says that Yusuke's time is up, and frees him to make his choice. Yusuke has no idea, but decides to hit Kuwabara—and unknowingly chooses correctly, as his body morphs back into Yana's form. Yusuke's reasoning was simple: he thinks that neither Hiei nor Kurama would be dumb enough to get caught (Kurama asks Hiei if his ears are burning), and that it'd be cheap to go after Botan, who is the only non-fighter of the group. Also, Kuwabara is the one most used to getting his ass kicked by Yusuke. Kido compliments Yusuke, and calls out the mastermind behind the ordeal…Genkai.

Genkai wanted to show the four that despite their strength, there were still people who could kill any of them if they weren't careful. She points out Hiei in particular before moving on to Yusuke, who keeps saying he wouldn't have gotten caught had he known the powers, which Genkai points out as being stupid, because what enemy is going to reveal their powers like that. She also reveals that there is more to the situation than just a life lesson, as Kido, Kaito, and Yana's powers are only a month old, and they are only a few of the many people who have come to Genkai seeking help with newly developed abilities. Genkai has realized the cause: someone is trying to open a hole to the Demon Realm.

Koenma then calls, and confirms what Genkai has said. He gives a bit more detail about the hole, and the stages of development. In order to be formed, at least one powerful psychic must sit in the center, and the hole must grow to have a two-kilometer diameter in order to be fully stabilized, and that will happen in about three weeks. The stages of development are:

1. The Human Realm becomes stagnant, and lower level life forms (such as the demonic insects) flourish.
2. Normal humans begin to develop spectral abilities.
3. Youkai of Class D or lower begin to thrive in the Human Realm
4. The hole reaches the two-kilometer mark, allowing class B and C demons to pass freely.

To give Yusuke a frame of reference, Koenma points out that Toguro the Younger was a B class demon. This completely shocks Yusuke, who had thought Toguro must have been the strongest demon type there was, and now learns there are many more just like him, in addition to classes that are stronger than him. Koenma reassures Yusuke, however, saying that in order to prevent A and S class demons from crossing, a barrier was put up in the subspace between the realms. Due to the expanse of the barrier, however, there are gaps that allow B class and lower youkai to pass through.

Hiei interrupts the talk to demand what his class ranking is from Koenma. He's unsatisfied, however, by his ranking of middle-B, and starts to leave. Kuwabara is outraged, and demands to know why. Hiei claims that he doesn't care about the humans, and that he wants to go back to the Demon Realm. He won't hinder Yusuke's efforts, but he won't help either. He goes, and Kurama says Hiei is conflicted because he does want to return. Yusuke remains confident that Hiei will show up in the end, when the others are helpless.

The rest of the group goes on without them, and learn that there are 7 enemies, lead by a former Spirit Detective named Sensui. Sensui was incredibly pure of heart, and naïve: he had operated under the belief that demons were evil while humans were good. On one case, however, he and his partner Itsuki came upon a group of humans that were torturing demons for the pleasure of it, their cruelty seemingly limitless. It destroyed everything he ever knew, and Sensui killed all the humans in the room. Shortly after, Sensui disappeared from the Spirit Realm's radar, taking with him a video called Chapter Black. Chapter Black contains every act of cruelty that mankind has ever preformed, including scenes from the Holocaust, Hiroshima bombing, and Vietnam War. The tape is said to run for hundreds of hours, though it would likely only take a few minutes of watching to turn a person against humanity.

Yusuke defeats one enemy called the Doctor early on, while Kuwabara battles a human named Mitari, who has the power to create creatures by mixing his blood with liquid. During the battle, Kuwabara reveals a new ability: the power to summon a sword that can cut through dimensions called a Jigan-to. This is especially pleasing to Sensui, who needs such a power to cut down the subspace barrier, and allow the most powerful demons to come through. Sensui and a human called Sniper attack the group, successfully separating them, and kidnapping Kuwabara.

Yusuke gives chase, but is stopped by Sniper, who uses his power to mark Yusuke's body with four bull's-eyes. Anything Sniper sends his ki into will now turn into a weapon, and without aiming, Sniper can toss it up into the air, and the item(s) will automatically try and hit the targets. The objects are small at first: leaves, rocks, knives. But then Sniper goes for something much larger: a fully loaded gas tanker. The truck barrels after Yusuke, who does fairly well in avoiding it…until Sniper fires a gun into the fuel tank, causing the truck to explode. Thankfully, Yusuke doesn't die, as Hiei has been watching, and rescues him.

Hiei defeats Sniper with great ease and skill, stabbing him in a way that Doctor would later describe as "threading" to avoid the internal organs. Yusuke, who is riled up with anger, starts to storm off, determined to take on Sensui and his group then. Hiei won't have it, however, and comes at Yusuke with his katana. Yusuke's off-guard, and Hiei traps Yusuke with the tip of his katana against the underside of his jaw, saying that it was the second time that day he's held Yusuke's life in his hands. Hiei and Yusuke begin to fight in earnest, without weapons. It's a pretty even match with punches being thrown back and forth. Yusuke released a greater amount of reiki as the fight continues, and ultimately makes Hiei bleed with a hard punch. Hiei's pleased by it, glad to see Yusuke's power hasn't completely weakened. He points out that it's Sensui's strategy to make Yusuke so frustrated, his power would be suppressed. Hiei starts to leave, but Yusuke wants his help, so he bribes Hiei by offering up the Chapter Black video. The two rejoin the others, and go to rescue Kuwabara.

The center of the hole, where the psychic is, is 550 meters underground, and it's decided that Yusuke, Hiei, and Kurama will go in first, leaving the rest to serve as a second wave should they fail. To get to it, the group must go through a maze of underground caverns. This is made significantly easier by the presence of Mitari, who has defected to Yusuke's side. The group makes it about halfway in, with Kurama leaving a trail of lantern plants behind, when they come to a door Yusuke recognizes from a video game. The child inside is named Amanuma, and his power is the ability to bring games to life. In order to enter and leave, the rules of the game must be followed. For this game, seven people are required, requiring the group to go back and get more players. They must achieve four victories to advance, which the group does, but at great cost: because he has taken on the role of the game's villain, Amanuma dies at the end, killed by Kurama. Before it comes to this point, Hiei tries to break out of the game using physical violence, but his efforts are useless. Seeing that he can't be of any benefit, since he doesn't know how to play video games, Hiei takes a nap until the game is decided.

The original group of four (Kurama, Hiei, Yusuke, and Mitari) continues on, while the others return to the entrance. They make it to the center cavern without further incident, and find Sensui and Itsuki watching a movie, with Kuwabara and another human named Gourmet in a boat in front of the opening to the Demon Realm. When Mitari points out that Itsuki should be sitting under the hole, Itsuki reveals that the hole has begun growing on it's own, and that he can no longer stop it even if he wants to. Gourmet steps forward to fight, and soon the group learns that Gourmet's ability is to devour the power of others by swallowing them whole. He's done this twice already, once to capture the mind-tapping ability of another human, and once to gain the body-morphing ability of Toguro the Elder. Kurama steps forward to fight, and ultimately traps Gourmet (whose mind has been completely taken over by Toguro the Elder) in a life sucking plant that will leave him to suffer for eternity.

Kuwabara is removed from the boat, and placed onto the shore by an unknown force. When the group rushes over to him, they're swallowed by a large shadow face, which is actually a demon called an Uraotoko. The demon spits out Yusuke into the lake, but holds the others prisoner within it a subspace dimension. Itsuki joins the group in the Uraotoko, and reveals himself to be an incredibly rare Yaminade demon, with the ability to travel between dimensions. Itsuki reveals his utter devotion to Sensui, even since their first meeting. The story of how Itsuki let Sensui descend into madness, and even enjoyed it, angers both Kurama and Hiei, but the cannot attack without being trapped within the Uraotoko (Itsuki must command the demon to free them).

Sensui and Yusuke begin to fight, with Sensui going over various reasons why Yusuke will die. Sensui's shirt is removed at one point, revealing a built body of scars that he claims were all inflicted through his training. He gets a bit more serious at that point, launching an attack of multiple ki balls at Yusuke. The volleys are rapid, with Sensui having more than enough energy to launch multiple attacks with ease. Yusuke, however, says the power of the attacks is weak, and charges through the moves. Instead of attacking, however, Yusuke begins moving around erratically to disprove Sensui's theory of being able to read his moves, even taking a swim in the lake. When he comes at Sensui again, Yusuke rips off his wet shirt, and uses it to bind his and Sensui's wrists together, hindering the man's fighting style. Yusuke gets on top of Sensui, and is pummeling him with punches, until Sensui suddenly fires a bullet through Yusuke's stomach. Sensui apparently has had a gun barrel installed into his arm, which can fire bullets of Reiki.

Sensui begins to mock Yusuke, and notably his personality and tone of voice have both changed. Itsuki reveals that Sensui has seven multiple personalities, and while Yusuke has been fighting Minoru until now, Kazuya has stepped up to take control. Kazuya begins to torture Yusuke, shooting multiple bullets into his limbs. Kuwabara tries to summon the Jigan-to in order to intervene, but his ability isn't fully developed, and he can't tap into the power. Desperate, Kuwabara tries to attack Itsuki with his Rei-ken, but is stopped by Hiei, who knows it'll do no good. Kazuya decides he's had his fun, and puts the barrel at Yusuke's throat, when Koenma shows up.

Using the distraction, Yusuke tries to shoot Kazuya with the Rei-gun, but Kazuya notices, and throws Yusuke aside so the blast misses. Koenma then steps forward, saying he will stop Sensui no matter what it takes. Koenma wants to talk to Shinobu, the original personality of Sensui who Koenma knows, but Shinobu doesn't want to talk, according to Kazuya. Koenma realizes that Shinobu can still hear, and tries to reach out to him. He removes his pacifier, which is actually a tool for storing spirit energy that can create the Mafukan, the most powerful type of barrier in existence that can even contain the S Class. Koenma will use the Mafukan to close the hole and trap Sensui if Sensui doesn't surrender. The personalities consider this, but decide to just kill Koenma instead. Koenma starts to activate the Mafukan, but is stopped by a reawakened Yusuke, who is pissed that Koenma stepped in to one of his fights. Yusuke takes the pacifier, and punches Koenma away when he tries to get it back. He then immediately demands that Kazuya go get someone harder, because Kazuya is too weak to waste his energy on. After a show of force from Yusuke, Shinobu takes over, and offers to shake Yusuke's hand. Yusuke refuses, and is easily overwhelmed by Sensui, who knocks him to the ground, then picks up the boy's limp hand and shakes it.

Sensui takes a break to get fresh clothes and replace his right hand, which was destroyed by the gun. It gives Yusuke a bit of time to recover, but to no avail, as Sensui begins to power up. The energy Shinobu uses is neither reiki nor yoki, but an incredibly hard to achieve power called seikoki, or "sacred energy." Yusuke is completely overpowered, and defeated without much of an effort on Sensui's part. The power is easily enough to kill everyone, Sensui possessing the strength of an S Class demon, and making Hiei visibly nervous, which hasn't happened in the series. Yusuke loses his grip on the pacifier at one point, and when Sensui tries to catch it, Koenma uses his energy to activate the Mafukan. Sensui is able to overcome it, however, redirecting the energy that would trap him and sending it shooting up through the earth, where it powers through 550 meters of solid earth to blast out the surface.

Yusuke knows that he's going to lose, but he still wants to defeat Sensui. So he provokes the man, telling him that something incredible will happen if he makes the kill, and that dying is part of Yusuke's strategy. The trapped group realizes that Yusuke plans to die so the others can reach their potential through grief, such as what happened to Yusuke in the Dark Tournament. Kuwabara begins to plead with Yusuke to think of something else, while Hiei and Kurama threaten Itsuki, demanding release so they can join in the fight. Itsuki refuses, saying that even though they couldn't win, they could possibly escape and become stronger. Itsuki knows that both Hiei and Kurama were once A Class Demons, and could regain that lost power. And Kuwabara still can't summon his Jigan-to, which would free them.

Yusuke comes at Sensui, who attacks. In a moment of total desperation, Kuwabara manages to summon the sword, and to free them…but not in time to save Yusuke's life.

In their rage, Hiei and Kurama's powers rise, both hitting A Class levels. Hiei launches the Dragon of the Darkness Flame at Sensui, who is protected by an aura of seikoki. The Dragon carries Sensui through the opening to the Demon Realm, and the others follow without hesitation. Because his energy isn't yoki, Sensui can pass through the barrier in subspace. However, only Kuwabara would be able to follow him through, since Hiei and Kurama are now too strong. Kuwabara slices through the barrier without hesitation, and all four continue on to the Demon Realm.

The four ultimately land on Guillotine Hill, and Sensui promises to fight in earnest out of respect for their effort. Hiei summons a second Dragon, clearly not thinking of the consequences to himself, and devours this one for it's nutritive effect. Hiei, Kurama, and Kuwabara all attack, but are easily and repeatedly beaten down. Sensui notes their strength, and says because of that, their deaths won't be easy.

Meanwhile, back in the human world, it's revealed that Yusuke is a Mazoku, and because he was so powerful when he died, he'll be reborn as a demon with even greater power. Koenma is thrilled, but agents of the Spiritual Defense Force try to destroy Yusuke's body before that can happen. Their efforts fail, however, and Yusuke does indeed come back to life. He and Koenma (who is defying his father's orders) ride Yusuke's transformed spirit beast Puu into the Demon Realm. When they catch up to the others, Yusuke gives them a hard time about trying to finish his battle. Hiei and Kurama see Yusuke for what he is, and have a rather good laugh over it. Hiei then says he's ready to fight Sensui again, as does Kuwabara. Sensui notes that all are growing even stronger in their relief, which is an unusual quality. Yusuke says he needs to be the one to fight Sensui, and Hiei lets him do it, vowing to catch up to both Yusuke and Sensui soon.

Yusuke and Sensui fight while the others watch from a safe distance. They both have S Class power, but Yusuke's perceptions haven't caught up to his new strength, making his fighting sloppy. They settle into a deserted area for the final fight, and the others note that Sensui's hatred seems to be fading, and he seems happy to be able to fight at full power.

Yusuke beats Sensui rather badly, ending the battle with a devastating Rei-gun blast. Strangely though, Yusuke is pissed off over it, immediately dashing to Sensui's fallen body and demanding he get better. Itsuki shows up, and reveals that Sensui had a terminal disease, and would have died in two weeks anyway. Yusuke still wants to fight, and Hiei says that's typical before he passes out from the strain of the Darkness Dragons. Sensui dies, and Itsuki takes his body to keep in a dimension where only they will be.

Everyone returns to the Human Realm, and within a couple of weeks, things seem to be returning to normal. Hiei does indeed get the Chapter Black video, but instead of watching or selling it, he destroys it. And it's from this point I'm taking Hiei from.