Hiei is the offspring of a mating between a Koorime (Hina) and an unknown fire demon. Hina is born a Koorime, a race of self-reproductive ice demons who are all female. She fell in love with a fire demon and bore his son. However, this kind of action is considered a crime in the Island of the Koorimes, which floats above the Makai. Thus because of this and the fact that he was a male, he was given the title, "The Forbidden Child", and as an infant was thrown overboard by Rui, Hina's best friend. Before she did, however, she told the infant that if he must, he should come back and take revenge upon the Koorimes. Hina shed two tears which turned into gems, one for Hiei and one for his sister, Yukina, who is full fledged Koorime. Then he was thrown off.
A band of thieves picked up Hiei in the forests of the Makai and took him in as one of their own. He learned survival skills, thievery and such. As he grew more and more powerful, the thieves finally left him, perhaps out of fear, for at the age of five he was an A class demon. He carries his katana (sword) everywhere as means of weaponry. His only pursuit was strength and his long lost sister, Yukina. He also looks for his tear gem, which he lost during a fight when the chain was broken, causing it to fall into a rushing river.
As Hiei is retelling the story of his past, he speaks of how he used the Tear as a lure to draw other demons to battle him so he could kill them and grow stronger. During one particular battle, the Tear was cut from his neck and lost in the valley below while he continued the duel on the cliff and killed his foe. Afterwards, he rushed to the floor and hunted for it, but it couldn't be found.
When he learned that Yukina had left the island (which was destructing) to search for him, he went to a demon named Shigure. He told Shigure his story and asked for a 'jagan,' a third eye to be implanted. Shigure did just that, and Hiei's power sunk, making him no longer an A class demon. Hiei gained back his power when he learned the jagan's techniques, such as the Kokuryuha (the Black Dragon Wave) and what we call Majin Hiei, which is his transformation process where his skin turns green and numerous eyes appear all over his body. Shigure had a thing for irony, and demanded that his price would be that Hiei could never tell his sister he was her brother, because hunting for her was the purpose of the Jagan Eye. But then, Hiei may have wanted to protect her from the truth of what he really is, anyway.
Hiei returned to the Island of the Koorimes, where he found Rui by Hina's grave. She told him Yukina had left to find him. She said he could kill her because Hina had committed suicide due to the loss of her son. Hiei didn't do a thing, but disappeared.
Hiei was not able to find Yukina in the Makai, so he left for the Nigenkai (Human World). There he met a boy named Kurama. Kurama was walking with a girl named Maya, who confessed to Kurama that she loved him dearly. Hiei attacked Kurama, thinking he was the demon who stole his tear gem or a spy of a demon Hiei was hunting. Hiei was injured during the fight, and Kurama took him home and nursed him. Kurama asked for his name, and reluctantly he replied. Then he left to continue on his missions.
A year or two later, they met again when Hiei asked Kurama to steal three Reikai artifacts with another demon named Gouki. Hiei stole the Shadow Sword, which can turn a person into a mindless demon if cut with it. Hiei tried this with Keiko, someone Yusuke cares deeply for, in an attempt to get the other two artifacts, the Orb of Baast and the Forlorn Hope. In the handle of the sword is the antidote to it's power, which cured Keiko. Gouki was killed by Yusuke, while Hiei and Kurama's punishment was to join Yusuke on his missions. Another human named Kuwabara also joined the team. Hiei had no choice, for if he kills a human, he will be severely punished or suffer death, as killing humans in forbidden by all means in the Reikai (Spirit World).
One of the biggest Yu Yu Hakusho events is the Ankoku Bujutsukai, a tournament on Hanging Neck Island in Ningenkai, where the reward is any wish granted. Hiei's team, the Urameshi team, won. Yusuke wished for Genkai to be resurrected (after Toguro Ototo killed Genkai, he told Koenma to keep her body and resurrect her if Yusuke wins). Soon after, another tournament arose. This time, it was with the three overlords of the Makai, the three most powerful demons. Their names were Yomi, Raizen, and Mukuro. It turns out Raizen was Yusuke's ancestor, and for survival he needs to feed on humans. Raizen refuses to do it, and soon after his death Yusuke took his place. Kurama joined Yomi, and Hiei became the heir of Mukuro. Before he left the Nigenkai, he was given a tear gem from Yukina. She indirectly called him "brother," and explained that it was because he would probably be what her brother would be like. He strings the gem into a pendant and left for the Makai.
There he met Shigure who challenged him to battle, pitted by Mukuro. The masked Mukuro said that he will reveal his true identity to whoever wins. Shigure said that if Hiei wins, he could tell Yukina who he is. Shigure knocked Hiei into a coma-like state of unconsciousness, but not before Hiei sliced through Shigure's forehead. Hiei had no problem with death, as he already had achieved all he wanted in life. Mukuro wanted to keep Hiei around, so she put him in a rejuvenation chamber where he slept until he regained his arm and his midriff wound was healed. She showed him her identity: a female whose body is half gone and replaced by metal and machinery. Hiei stayed around with Mukuro and whatever happened afterwards is a mystery. Hiei's story is perhaps the saddest one in Yu Yu Hakusho, where his life and soul was constantly challenged by betrayal, angst, and his disturbing past.
Yuusuke's mother was a drunk.
Kuwabara wasn't strong enough.
Kurama's best friend died.
Hiei. No, Hiei's whole life was screwed.