Once, in the Koorime islands -- a place where people                                                    were all Ice Demons... A koorime named Hina gave                                                    birth to two babies. One is an ordinary and healthy                                                    young baby girl but the other was said to be forbidden.                                                   The baby boy was a fire demon. The elders said that the                                                    boy would bring destruction to their place and they                                                    should eliminate the child. Hina didn't agree, she doesn't                                                    want her child to be thrown down the floating island but                                                    she couldn't do anything. The elders ordered Hina's                                                    best friend, Rui to do the job of getting rid off the fire                                                   demon. Hina produced two pearls for her two children,                                                   one for the girl and for the boy. Before Rui would throw                                                   the boy away, she gave him the pearl and told him,                                                   "Promise me you'll survive and return to this Island.. and                                                   take my life and destroy this island..." And the boy was                                                   thrown down the island. The forbidden child was known                                                   as Hiei.                                                   One time, a bunch of thieves returned to their hideout                                                   when one of them saw Hiei floating on the river. They                                                   took care of him and as he grew, he became one of them                                                   but a better thief. He likes to see blood on his blade/                                                   katana and fond of hearing anyone shrieking in pain or                                                   anyone dying. Everyday of his life, there's someone who                                                   wants to kill him, take his life but they always failed.                                                   When he grew up, the thieves that took care of him                                                   got scared of him and didn't want his company anymore.                                                   They said that he was a ruthless and heartless demon,                                                   that I day he would kill them all. He knows when he is                                                   not wanted so he left them, that's why he's a loner. The                                                   pearl that her mother gave him made him calm and made                                                   him feel safe.At first, the only thing he wanted to find was                                                   the place of the Koorimes. But the youkais that wanted                                                   him dead, fought seriously. His pearl fell to the cliff and                                                   that's the second thing that he wanted to find.                                                   In Makai, he heard that there's a doctor that can give                                                   him a third eye --- JAGAN ---, it is an evil third eye that                                                  allows him to see things that ordinary youkai can't see.So                                                  he finds Shigure that helped him out. When he got the                                                  Jagan, he quickly searched for the Koorime island and                                                 when he found it, he went there.When the Koorimes saw                                                 him,everyone got scared and nobody wants to talk to him                                                 or see him. On his way, he found Rui, the one that threw                                                 him out.He swore to kill all the koorimes but when he                                                 saw Hina's grave and heard Rui said that Hina had a                                                daughter named Yukina. That she disappered few years                                                ago to find his twin brother and since then they never                                                heard news about her. He changed his mind..                                                When he knew that Yukina's his sister, he quickly                                                searched for her. He thought that she might be in                                                Ningenkai. And there, he crossed-path with Shuichi                                                Minamino-- the fox thief that got wounded in Makai                                                (Youko Kurama) that's hiding in a human body. And his                                                story goes on when he and Kurama and a youkai joined                                                forces and get Reikai's three valuable things : a sword, a                                               mirror and a ball-like thing. He fought with Yusuke                                               Urameshi but he was defeated and then on, he made                                               friends with him and became one of the Reikai Tanteis.                                               One of their missions, was to save a young girl named                                               Yukina. At that time he saw her sister but never told her                                               the truth-- that he was her brother.                                                  As times passed, Hiei and the gang fought their enemies                                               one by one until one day , the three powerful beings in                                               Makai send them invitations to join them. Yusuke went                                              first to his father RAIZEN. Hiei went to see Mukuro.                                              At first, he hated her so much but after a fight with Shigure                                             that he almost died, thanks to Mukuro, he still lives. He                                             became Mukuro's right-hand man. And from that day on,                                             he's always with her. At the Makai tournament, Hiei fought                                              with Mukuro but had been defeated. At the end of the series,                                             he stayed in Makai with Mukuro.                                                                     ( I just don't know if he still goes back to Ningenkai to see Yukina )