Kohaku is the younger brother of Sango who was resurrected by Naraku after he killed his father by possessed by web. Naraku brainwashed Kohaku, taking his humanity away using a shard of the Shikon no Tama, which was embedded in his back to keep him alive.
Kohaku lived with his sister Sango and their father, in a village of demon slayers. Eventually, his father decides that he is old enough to become a demon slayer himself, and while he is reluctant, Sango helps to alleviate his fears. They arrive at the castle they are assigned to, but unknown to them, Naraku has taken the form of the castle's lord, possessing Kohaku and forcing him to kill his own father. He dies just before he can strike a blow at Sango.
After Sango joins Inuyasha's group, Naraku resurrects Kohaku, removing his traumatic memories and brainwashing him to be an emotionless killer. He is granted the power of a Shikon Jewel Shard, which enables him to become more proficient with his weapon. At first, Naraku uses Kohaku to coerce Sango into surrendering the Tessaiga, and during their confrontation, Kohaku is present. He briefly regains his memories, only to have them removed again. He encounters his sister multiple times, and each time he claims that her face seems familiar even though he cannot remember it.
Later, when Kagura abducts Rin to lure Sesshomaru to Naraku's castle, Kohaku is tasked with standing guard over Rin. When Naraku orders him to kill the girl, he lures her out into an open field away from Sesshomaru to do the deed. When Inuyasha's group and Sesshomaru find them, the latter is intent on killing the demon slayer, but when he confirms Rin's safety he gives up.
During the Band of Seven arc, he served as a messenger and liaison between Naraku's incarnations and the band of mercenaries. He also guides Sango through Mt. Hakurei using a luminescent cloak. After Hakudoshi's birth, he and Kanna are charged with caring for The Infant. They travel to a castle where one of the women has a stillborn and entrust The Infant in her care, after Kanna tells her that her child is alive and wipes her memories. Meanwhile, Kohaku becomes good friends with the inhabitants of the castle, but when Princess Abi attacks, Naraku orders the demon slayer to kill everyone within. He is eventually forced into it, and the last woman left alive notices that he is doing it against his will before she herself is slain.
Having spent a great deal of time with the people at the castle, Kohaku finally regains his memories. He throws himself off of Kagura's feather in an attempt to commit suicide, but Kagura swoops underneath him before he hits the ground. His suicide attempt unsuccessful, Kohaku resolves to redeem himself by fighting Naraku, while at the same time feigning loyalty to him, meaning he still had to follow all his orders unquestionably.
When he and Hakudoshi were tasked with luring Kikyo out with a swarm of demonic rats, Kohaku is ordered to leave the doors to Zushinezumi's shrine open, putting his plan to the test. Sango encounters him, and because of the gravity of Naraku's deeds, decides to kill Kohaku. She is stopped, and the shrine is destroyed, ensuring that Kohaku could continue his ruse. After Naraku acquires the Fuyoheki and gives it to The Infant, Kohaku is tasked with slaying strong demons.
Meanwhile, Hakudoshi formulates his own plan to betray Naraku, and enlists Kagura's help, requiring shards in order for his plan to work. She attempts to kill Kohaku, but eventually betrays Hakudoshi and protects him, ordering him to flee while she deals with The Infant and his co-conspirators. Hakudoshi's demon golem, Moryomaru, tracks the demon slayer down, but Inuyasha's group intervenes. Kohaku finally reveals to Sango that his memories have returned, and he openly betrays Naraku and begins traveling with Kikyo after Kagura's death. After Kikyo's death, Kohaku joins Sesshomaru in his pursuit of Naraku, forming a close bond with Rin. Eventually, his shard is removed by Naraku, killing him, but Kikyo's light resurrects him. He does not openly combat Naraku during the final battle, but protects Rin by giving her his mask.
Three years later, he is seen traveling with Kirara and training to become a demon slayer.