Hiei is a fire-apparition and the only born son of Hina, a Koorime [ice-apparition] woman. Hiei was born of what were considered to be "taboo" methods on conception on the Isle of Koorime. Koorime apparitions are all females and conceive one child ever 100 years through asexual reproduction. The child is always female, so long as the mother remains "pure" and "untainted" by the ways of men (i.e. a virgin). If a Koorime woman becomes pregnant by leaving the floating island of Koorime and having sex with a man, then the child will be born a male (or in the case of Hina, twins were born. One boy and one girl).
Because the Koorime consider men to be violent, evil and murderous, Hiei was shunned at birth and intended to be executed for being born a male. He was cast off of Koorime Island, reluctantly, by his mothers best friend, Rui, and left to fall to his death in the forests of Makai, alone. By Hiei's own words, he
4'10 without his hair [as billed by Koto in the dark tournament.] (147cm.)
"willed himself to survive," if for nothing else than for the sake of one day returning to Koorime island and extracting his bloody, murderous revenge on every single living Koorime.
Despite being born from a mother who was an ice apparition, Hiei is clearly a fire-apparition. As he states in his battle with Mukuro, he considers it to be his "curse." "To be born with a nature that is in direct opposition of my breed. With no choice but to consume everything around me with the flames of my hate."
After being cast off of Koorime Island, Hiei was discovered and there-after, raised by a group of bandits. Even from a young age, as a child, he had an innate desire to inflict pain on others. Taking great pleasure in spilling the blood of his enemies and pursuers whenever given the opportunity. Because of his incredibly violent tendencies, Hiei was eventually ostracized by the rest of the bandit group, for fear of being killed by him. Finding himself rejected by yet another "family," Hiei left and from then on, lived and survived on his own.
Eventually Hiei sought out the demon surgeon, Shigure, in order to have a Jagan Eye surgically implanted into his forehead, for the sake of finding his way back to Koorime Island and for the sake of finding the Hiruiseki stone (tear gem) from his mother that he had lost in a battle against another demon. After telling Shigure his life story and proving his tolerance for pain, Shigure agreed and gave Hiei the Jagan eye that he desired. This is where Hiei is given the title of "Jaganshi Hiei," or "Hiei of the Evil Eye." (It should be noted that "Jaganshi" is NOT Hiei's 'last name.' It's only a title he was given after he became infamous throughout Makai. Hiei has no known last name.)
Eventually, Hiei returned to Koorime Island and had intended to kill every living Koorime on the island, but he discovered them to be reclused, shut in and fearful of any being from outside their island. He deemed them unworth killing, as they were, "already dead." In talking to Rui, who had no idea, initially, who he was, he learned that his mother had died several years earlier, having commited suicide because of the grief that she felt from the death of her son. Rui also informed Hiei for the first time, that Hina had given birth to another child, a girl. Upon learning this, Hiei's decided his new quest was to seek out his sister, Yukina. And he left Koorime island, never to return.