Jaganshi Hiei, whose name is often translated as Hiei of the Evil Eye, is a central figure known for his origins as a fire demon born to the all-female race of ice maidens known as the Koorime. His birth was a forbidden event, as male children were prohibited in their isolated society. Conceived when his mother, Hina, interacted with a male fire demon from outside her community, Hiei and his twin sister Yukina were born. Because of his male gender, he was deemed a cursed child, and as an infant, he was cast out from the floating ice country and thrown off its precipice to his assumed death. He survived this fall and was later found and raised by a group of bandits in the demon world, where he learned to kill at a very young age. His mother, overcome with grief, later committed suicide, but not before shedding two Ice Tear stones, one of which was given to Hiei at his abandonment. This jewel became his sole source of comfort and sanity throughout his brutal childhood, until he lost it during a fierce battle.

This loss of his mother's tear stone became a primary motivation. To locate the precious jewel and find the ice village of his birth, Hiei sought out a demon surgeon named Shigure. Hiei underwent an excruciating procedure to have the Jagan, or Evil Eye, implanted in his forehead. The operation was so demanding that it drained him of his original A-class demon power, reducing him to a lowly D-class. As payment for the surgery, Shigure took a piece of Hiei’s life story and later made him swear an oath never to reveal his true relationship to his sister Yukina. With his new Jagan eye, which grants abilities like telepathy, mind control, and clairvoyant remote viewing, Hiei quickly located the ice village. There, he learned of his mother’s death and the existence of his twin sister, who had left to search for him. His quest then shifted from finding his tear stone to silently watching over and protecting Yukina from a distance.

In the story, Hiei is first encountered as a formidable antagonist. Alongside thieves like Kurama and Gouki, he steals three treasures from Spirit World, one of which is the Shadow Sword that can turn humans into demons. He captures the main protagonist Yusuke Urameshi’s friend, Keiko, leading to a confrontation. After being defeated, he is forced to work as a temporary ally to Spirit World to pay for his crimes. His role gradually evolves from a villain and reluctant ally into a core member of the team, known for his ruthlessness in battle, where he dispatches enemies with lethal efficiency and often makes good on his threats to kill. Hiei typically keeps a distance from others and is cynical and taciturn, often belittling human emotions and the importance they place on friendship. He frequently squabbles with the human Kazuma Kuwabara, mocking his loud personality and lack of skill, though a grudging respect for each other’s strength develops between them over time.

Despite his cold and aloof exterior, Hiei forms a deep, unspoken bond with his teammates. He is closest to Kurama, his former partner in crime, with whom he shares a history and a more even-tempered rapport. He also shares a complex rivalry and mutual respect with Yusuke, recognizing his power and determination. The most significant relationship in his life is with his twin sister, Yukina, though she is unaware of their connection. In her presence, he displays a surprising and uncharacteristic degree of sincerity, warmth, and protectiveness, even going so far as to spare the life of a human who imprisoned her simply because killing him would make her sad. Later in the series, Hiei comes under the tutelage of the powerful demoness Mukuro, whose strength he greatly respects. He chooses to remain by her side, and a bond forms between them as he helps her confront her traumatic past.

Hiei’s development throughout the series is marked by his gradual, reluctant acceptance of companionship. Having struggled his entire life with the pain of abandonment and his identity as a loner and killer, he finds himself fighting alongside others and, despite his protests, coming to their aid. He learns to accept his place within the group, even as he maintains his solitary demeanor. His journey also involves mastering his own immense power, particularly his signature technique, the Dragon of the Darkness Flame. This move summons a powerful black dragon made of demon fire, which he can either unleash upon his enemies or, with great risk, consume to dramatically enhance his own physical abilities. For much of the series, he keeps his right arm bandaged to contain this power. After gaining full control over the dragon, the bandages are removed to reveal a dragon tattoo covering his arm. He is a highly skilled swordsman, a master of the Sword of the Darkness Flame technique, and his small, lean stature belies his incredible speed and explosive, fire-based attacks.