He is the son of a koorime or an ice princess.  Ice princesses are a whole race of females who live on a floating island of ice (kind of like Laputa).  So if a son is born, he's immediately kicked off the island in fear that he could destroy the entire race.   Since Hiei is not only male but also a fire demon, he was dropped off a cliff despite his mother's protests.  He knows all this so he's extremely bitter and decides to destroy the entire race.  However, he changes his mind because the whole race is almost destroyed by the time he gets enough power anyway.  Every time a koorime gives birth, she'll cry a special tear gem for each child.  Yukina and Hiei both got one, but Hiei was raised by bandits and he lost his during his travels.

Even though he keeps on saying Yusuke's an idiot, etc. he still helps Yusuke to fight youkai sometimes. In fact, he actually changes as he hangs around with Yusuke, Kurama, Kuwabara, and their friends more. He becomes less cold, doesn't kill everything he sees, and even protective of Yusuke when he thought Sensui had killed him. When Mukuro, one of the rulers of Makkai, invites him to Makkai he becomes even more powerful (up to an A-class youkai). She also returns to him his lost tear gem.   During the tournament to decide the future ruler of Makkai, though, he still loses to Mukuro.  In the end, he stays with her to help lost humans in Makkai go back to Ningenkai.  Hiei is sometimes thought to be Mukuro's lover but I bet many fans of the Kurama-Hiei relationship will argue against that.  However, the picture on the bottom left may change some minds.

Seiyuu: Hiyama Nobuyuki: Now people who know Hiei fairly well should visit this page. It's quite interesting to see that the voice of a cold, frowny-face youkai would be a happy-smiley man.