Kohaku, the younger brother of Sango, belongs to a lineage of demon slayers, the yōkai taijiya. His childhood ended tragically when Naraku orchestrated an ambush that slaughtered their clan, including their father, and resulted in Kohaku's death. Naraku revived him using a Shikon Jewel shard embedded in his back, erasing his memories and transforming him into a manipulated puppet forced to commit violent acts, such as attempting to steal Inuyasha's Tessaiga. Encounters with Sango gradually restored his lost memories, plunging him into profound guilt over his actions under Naraku's control. Kohaku eventually rebelled, feigning continued obedience while secretly working to undermine Naraku's schemes. Following Naraku's defeat, the Shikon Jewel shard was safely removed from his body, his life preserved by Kikyō's lingering spiritual energy.

In the years after Naraku's downfall, Kohaku travels independently as a demon slayer, accompanied by the twin-tailed cat demon Kirara. He acquires a new, heavier weapon crafted by the swordsmith Tōtōsai. While he rarely visits his sister Sango due to her busy family life, approximately six months into his travels, he encounters Sesshōmaru and learns of a demonic threat named Root Head. Kohaku aids Miroku in combating this demon and later babysits Sango's infant son while she fights.

By the time of the Yashahime events, Kohaku has matured into his early thirties and risen to become chief of a new generation of demon slayers, succeeding his deceased father. He bears a distinctive scar across his nose, sustained while protecting his nephew Hisui from a praying mantis demon attack. Leading a group that includes Hisui and Sesshōmaru's daughter Setsuna, whom he personally trains, Kohaku emphasizes strategic retreat as a valid tactic. This pragmatic approach contrasts with the aggressive methods of younger slayers like Towa. Kohaku demonstrates his expertise during sparring sessions, reinforcing that tactical withdrawal is not weakness.

Physically, Kohaku evolves from a preadolescent with spiky dark brown hair, freckles, and a topknot into a taller adult with faded freckles and his characteristic nose scar. His attire changes from a sleeveless lavender kosode with demon-slayer armor to a long-sleeved lavender kosode paired with a brown vest, black pants, and geta sandals. He retains his signature armor but adds a short black-and-red shoulder cape secured by a yellow cord.

Kohaku's personality undergoes significant transformation. Initially kindhearted yet crushed by guilt and manipulation, he develops into a confident, strategic leader. His experiences forge resilience and a strong sense of responsibility, particularly towards mentoring younger demon slayers. He maintains a pragmatic outlook, prioritizing mission success over personal glory, and exhibits dry humor regarding his avoidance of Sango's crowded household. Despite emotional scars, Kohaku demonstrates unwavering dedication to protecting others, embodying the disciplined yet adaptable ethos of a seasoned demon slayer.