Hiei generally comes off as having a fairly nasty disposition, as he's completely unafraid to say things in the most blunt or insulting manner possible. He has an ego that by far dwarfs his height, and it shows in his actions: in one battle, Hiei lost his soul because he thought a certain game was beneath him, and purposely broke the rules of it. A one-time villain, Hiei has no desire to be the hero, and acts in his own interests, often needing to be bribed before he'll come along with the team. And he certainly isn't at all restrained when it comes to the threats, promising death to many who would come against him, and even looking forward to the kill.

So. Smug, self-serving, short-tempered, and violent. But there's more to him, or else this would be a night shift application.

Over the course of the series, Hiei changes from a typical anime villain who wants to destroy all humans to an odd balance of compassion and mercilessness. His nature loses much of its cruelty even early on, when Hiei insists on fighting Seiryu, who just killed one of his own allies. Kurama notes that Hiei would have done the same once, but has changed enough to look down on Seiryu's actions with disdain. And despite his deep dislike for Kuwabara, when Hiei trains him, he holds back because of Kuwabara's weakness.

Of course, Hiei never actually verbalizes any of this. When thanked for saving the group at one point, he says it's only because the greater numbers is in his own benefit, which is interpreted by Kurama as a way of saying "you're welcome." Such is a pretty typical response: innocent questions are answered with harsh, insulting comments, some of which are actually quite helpful. It just isn't Hiei's personality to say it in a sweet way.

And despite his selfish nature, Hiei does indeed care about others, and will act to help them…sometimes. Typically this happens when one of his friends is completely helpless: at one point Hiei is ready to fire should a fighter kill the unconscious Kurama, despite the rules of the battle they are engaged in. Later on, when Yusuke is too enraged about the progress of a battle to act wisely, Hiei fights with him to calm him down. When Yusuke dies, Hiei is the first to attack the killer in fury, and carelessly risks his life by using the Dragon of the Darkness flame technique twice in the battle. And Hiei would do just about anything for his younger sister, Yukina. Of course, there are exceptions to all of this: when told about the impending threat of a tunnel opening to the Demon Realm and ensuring the end of the human race, Hiei leaves the group to their own devices, offering neither help nor hindrance. When Yusuke and later Kuwabara get kidnapped, Hiei says it's what they deserve for their carelessness, and refuses to step in until he is promised something in return. And despite knowing that Yukina wishes to know whom her older brother is, Hiei refuses to tell her, never having any intention of doing so.

And then there's the less seen, utterly adorable naïve side of Hiei's personality, entirely unexpected when confronted with his cool behavior. Because his childhood wasn't happy or even normal in the least, Hiei doesn't know certain things that the others do, like how to play Rock-Paper-Scissors. Even though it's completely trivial, Hiei is still flustered by not knowing how to do it (and calls it pathetically simple when he's shown). He also takes a good deal of teasing about his sister, particularly from Kurama and Yusuke, who both know the truth, but still enjoy seeing Hiei "blush," which amounts to him scowling and growling out a threat to kill them all. Kurama also ribs him about the arrogant mistake that cost him his soul, getting a similar reaction.

Ultimately, Kurama describes Hiei's personality best when he says that Hiei is a complex person.