Jaganshi Hiei, whose name means flying shadow, is a central figure in the narrative. He is a fire demon born from the race of ice maidens known as the Koorime, a society which exclusively produces female offspring through parthenogenesis. His birth was an anomaly, a male child resulting from his mother Hina's interaction with a male fire demon outside the isolated village. Because of this transgression, Hiei was deemed a cursed child and, as an infant, was cast out from the floating ice country and thrown down to the surface of the Demon World. His mother died not long after, having wept two tears of ice at the birth of her twins which crystallized into jewels; these gems were given to Hiei and his twin sister, Yukina, before her death. Hiei survived his fall and was found and raised by a band of thieves, learning to fight and kill at a very young age. During this time, he wore his mother's ice tear jewel around his neck. The jewel brought him a sense of calm, but he eventually lost it in a battle. This loss, combined with a desire for revenge against the village that rejected him, drove him to seek out the demon surgeon Shigure. Hiei underwent an excruciating procedure to have the Jagan, or evil eye, implanted into his forehead. This surgery granted him powerful psychic abilities like clairvoyance and mind control but at a tremendous cost, draining him of nearly all his original A-class demon power and reducing him to a lowly D-class apparition. To survive this weakened state, he trained under Shigure in swordsmanship, swearing an oath to never reveal his true identity to his sister. Using the Jagan, he quickly located the ice village but found his desire for revenge had faded, instead choosing to secretly watch over Yukina from that point forward.

In terms of personality, Hiei presents as aloof, cynical, and taciturn, preferring to keep his distance from others. He frequently belittles human emotions and the value placed on friendship and life, often engaging in heated squabbles with his teammate Kazuma Kuwabara. Despite his gruff exterior, he adheres to a personal code of honor and holds a deep, unspoken respect for those he fights alongside, particularly Yusuke Urameshi and Kurama, with whom he shares a strong bond as former accomplices. His most notable emotional vulnerability is reserved for his twin sister, Yukina, toward whom he shows surprising sincerity, warmth, and protectiveness, all while ensuring she never learns of their sibling relation, believing she would be happier not knowing. He also develops a profound respect for the demoness Mukuro, finding kinship in her strength and later choosing to remain by her side.

Hiei's primary motivation shifts throughout his life. Initially driven by a thirst for revenge and a desire to reclaim his lost jewel, his goal evolves into a quiet mission to find and protect his sister at all costs. His actions, though often appearing self-serving, are frequently guided by these deep-seated feelings of abandonment and a lone desire for connection.

Hiei's role in the story begins as an antagonist, having stolen treasures from Spirit World alongside Kurama and a demon named Gouki. He uses one of the artifacts, the Conjuring Blade, to cut Yusuke's friend Keiko, turning her into a demon. He is defeated by Yusuke with help from Kurama, and subsequently joins the team as a conditional ally to work off his sentence. He proves to be an invaluable and fearsome asset in the battles that follow, from the Saint Beasts arc where he ruthlessly dispatches the ice master Seiryu, to the Dark Tournament where he masters his devastating signature technique. While he initially hesitates to join the fight against the rogue psychic Shinobu Sensui, he eventually becomes a key player in the Chapter Black saga. In the final arc of the series, Hiei returns to the Demon World, becoming Mukuro's second-in-command and participating in the tournament to unify the three realms, where he ultimately loses to Mukuro.

Key relationships define his character arc. His bond with Kurama is one of mutual respect and deep understanding, as they are often allies in thought and action. With Yusuke, he shares a rivalry based on strength, acknowledging the spirit detective's power and growth. His bickering with Kuwabara masks an underlying trust and camaraderie, though he would never admit it. His relationship with Shigure is a transformative one, as he is the one who gave him the Jagan and taught him to wield a sword, setting the course for his entire future. The most significant relationship is with his sister Yukina; everything he does is colored by a desire to keep her safe and happy, even if it means remaining a stranger and a shadow in her life.

His development is marked by a gradual shift from a purely self-interested and ruthless killer to a more complex individual capable of loyalty and sacrifice, though he never loses his sharp edge or cynical demeanor. He begins his journey seeking power and revenge, but through his engagements with Team Urameshi, he finds a sense of belonging he never had, even if he would deny it.

Hiei is a formidable combatant, renowned for his incredible speed and swordsmanship. He wields his sword with deadly precision. His signature abilities stem from his demonic fire nature and the power of his Jagan. The Jagan grants him various psychic powers including clairvoyance, telepathy, and the ability to control the minds of weaker beings. His most famous offensive technique is the Dragon of the Darkness Flame, a powerful attack that summons black flames in the shape of a dragon to incinerate his opponent. Initially, using this technique severely damages his own arm, requiring it to be kept bandaged. He eventually masters the technique, learning to absorb the black dragon's flames to dramatically amplify his own power, speed, and stamina. He also possesses the Sword of the Darkness Flame, which allows him to shroud his blade in the same destructive black fire. As a fire demon, he can generate intense flames, such as with his Fist of the Mortal Flame technique. Possessing a natural talent for battle, his power class grows exponentially over the course of the series, beginning as a lower D-class and eventually reaching the highest levels of S-class demon energy.