Hiei is one of the most iconic characters in Yu Yu Hakusho, undergoing some of the greatest changes throughout the entire series. Starting as an early story arc's antagonist before teaming up with Yusuke and the others, Hiei's transformation from villain to hero to wannabe Lord of the Demon Realm is one of the most fascinating stories a rival character has ever seen in a shonen anime.
Looking at Hiei's arc over the course of Yu Yu Hakusho, here are the five events that most altered the beloved three-eyed demon forever.
When Hiei Was Abandoned By His Family
Hiei is a fire demon born to the isolationist tribe of Ice Apparitions. This all-female race avoided procreation whenever possible, but always selected to give birth to girls. Hiei's mother, however, met a fire demon and helped produce Hiei, which the Ice tribe saw as an abomination. As a result, the infant Hiei was dropped off a cliff.
Obviously, Hiei didn't die. He was rescued by bandits, which set his life down a dark course of violence and cruelty. However, his abandonment at the hands of the Ice Maidens resulted in Hiei's life taking a truly horrible path as he began his descent into villainy.
When Hiei Met Kurama
The partnership between Hiei and Kurama changed the trajectory of Hiei's entire character. The two first met in a manga one-shot that wasn't adapted in the anime until the Yu Yu Hakusho OVA was produced years later. Hiei and Kurama start off as adversaries, only to team up to combat another demon known as Yatsude. Hiei is convinced Yatsude has devoured his sister Yukina.
While Hiei is clearly untrusting of others, his partnership with Kurama, which takes place a year prior to his encounter with Yusuke, is ultimately the first stepping stone Hiei takes to working with others. Up to this point, Hiei had a reputation for killing anyone who dared to help him. However, Kurama is not only one of the few people Hiei opened up to at this stage of his life (albeit slightly), but someone he later trusts to aid him on his heist of King Enma's treasures.
When Hiei Was Defeated By Yusuke
Hiei's duel with Yusuke marked the start of Hiei's transition into heroism. While in a more basic sense it demonstrated that Yusuke was stronger, it also forced Hiei to grapple with two other, very important themes that would define the rest of his run on Yu Yu Hakusho. For one, Yusuke disproved nearly everything Hiei had believed up until that point. Hiei saw himself as a lone figure who could call upon maybe Kurama and other partners as need be. However, Yusuke's friendships ultimately led to his victory. This fact forced Hiei to reevaluate how he worked with others and saw other people.
For the first time since infancy, Hiei was at the mercy of someone else, but Yusuke and the Spirit Realm pardoned Hiei for his crimes. This mercy was the first event to really soften Hiei, which would become apparent going into the Four Saint Beasts arc, the Dark Tournament and beyond.
When Hiei Rescued His Sister Yukina
Yukina is arguably one of the first people Hiei has a fully tender relationship with. He cares for his sister unconditionally and protects her from afar. We first see signs of Hiei's feelings for Yukina when he meets Kurama, but it's near the end of the Spirit Detective Saga that we see him rescue Yukina from her kidnapper Gonzo Tarukane.
Hiei doesn't kill Tarukane -- the evil human is later dispatched by the Younger Toguro and Sakyo -- but he does express a fiery determination to save Yukina at any cost. From there he begins to open up more emotionally to Yukina, and his fighting teammates as well. This gradual openness proves vital as the series goes on, especially in the Three Kings Saga.
Hiei's Birthday Gift to Mukuro
Hiei meets Mukuro -- one of the titular Three Kings of the Demon World -- in Yu Yu Hakusho's final arc. Mukuro has had a very dark and twisted past, with her father abusing her violently. She stands as a kindred spirit to Hiei in many ways, the two sharing similarly abusive pasts. This ability to empathize with Mukuro allows Hiei to do something truly compassionate: give a gift.
In the anime, Hiei breaks the shackles that bind Mukuro, signifying freedom from her abusive past. In the manga, however, he gives her an explicit birthday present: her father, kept perpetually alive by one of Kurama's parasitic plants. Hiei gives her the plant so she can torture her father as he tortured her, offering her a chance at vengeance. In both cases, Hiei attempts -- in his own way -- to do something kind for someone he emotionally connects to. And both times, Mukuro is grateful for the gift.