Kohaku is a young demon slayer and the younger brother of Sango. He was raised in a village of demon slayers and trained from childhood to fight evil spirits. On his very first mission, he was manipulated by a spider demon acting under the orders of Naraku. Under this influence, he was forced to kill his father and fellow slayers before being mortally wounded himself. Naraku then resurrected him by placing a shard of the Shikon Jewel in his back, erasing all of his memories and turning him into an obedient puppet.

As a result of his manipulation, Kohaku initially exists in an amnesiac state, following Naraku's orders without question. However, as he repeatedly encounters Sango and the rest of the group, fragments of his painful past begin to resurface. He is haunted by deep guilt over the murders he was forced to commit and becomes fiercely motivated to destroy Naraku, often believing that sacrificing his own life is the only path to redemption. He is tactical, methodical, and possesses a strong, quiet inner resolve that allows him to hide his regained memories and rebel against his master from within. In The Final Act, he forms a secret alliance with the wind sorceress Kagura, who shares his desire for freedom.

Kohaku plays a central role in the emotional weight of the story, serving as Naraku's primary means of tormenting Sango. His Shikon shard becomes one of the last remaining pure fragments, making him a vital instrument in the plan to defeat Naraku. After Kikyo purifies his soul and imbues his shard with holy light, he begins traveling with her to maintain its purity. Following Kikyo's death, he is saved from the demon Byakuya by Sesshomaru, and he remains with Sesshomaru, Jaken, and Rin for the latter part of the series. His shard is eventually targeted by the evil entity Magatsuhi, who takes possession of Kohaku's body and forces him to relive the murder of his father. Kohaku ultimately overcomes this nightmare through his own willpower and the light Kikyo left behind, allowing him to regain control. In the final battle, he willingly offers his shard to Naraku under the guise of surrender, using the opportunity to strike at the villain with a sacred arrow.

Kohaku's most important relationship is with his sister Sango. Their bond is defined by tragedy, separation, and eventual reconciliation. He also shares a deep mutual understanding with Kagura, as they are both former puppets of Naraku seeking their own freedom. Kikyo acts as a spiritual guide whom he deeply respects, and Sesshomaru serves as a protector who saves his life and allows him to travel with his retinue.

As a demon slayer, Kohaku is highly skilled in combat. He is expert with a kusarigama, a weapon consisting of a sickle and a weighted chain, using it with agility and precision. Under Naraku's control, his combat abilities are heightened, but he retains his tactical intelligence. After finally freeing himself from Naraku's influence and becoming an adult, he takes responsibility for reforming and leading the demon slayer clan, displaying a sharp strategic mind. His life is sustained by the Shikon shard, but his real strength comes from his resilient spirit, which enables him to endure possession, face his traumatic memories, and choose to live fully rather than surrender to guilt.