Kohaku is Sango's little brother, When he was still an infant, his mother died and his sister took care of him along with their father.. He is also a yōkai taijiya, like his older sister and their father, from the Yōkai taijiya village. He was manipulated by Naraku through most of the time during the Shikon Jewel's return to the feudal era, and he was only kept alive by the jewel shard in the back of his neck.
He was killed by a demon posing to be a castle's lord during one of the Yōkai Taijiya's missions and was made to kill every Yōkai Taijiya that was there. He is later revived by Naraku as a human puppet and has a Shikon no Tama shard embedded in his back to keep him alive and under the control of Naraku without memory of what he had done. Kohaku, at first before being slain, was inexperienced with his weapon, a kusarigama (a demon bone sickle on a chain). He then becomes a rather advanced wielder with the power of the Shikon Jewel shard.
He encountered his sister a few times not long after being under Naraku's control and at first he was unable to recognize her at all, although her face does seem familiar to him. Early on, Kohaku was ordered by Naraku to kill Kagome, but he was prevented from doing so.
Kohaku is later used as a guard to keep an eye on Rin when Naraku kidnaps her; he was to kill her or be killed by Sesshōmaru, only to fail at that.
He later served as Naraku's messenger, along with Kanna for the Band of Seven. He encountered Sango again during this time, and protected her by leading her outside Mount Hakurei without revealing his identity. He is later charged with keeping the Infant safe, along with Kanna, by staying in a human castle. When the castle is attacked by Princess Abi, Kohaku fights against Naraku's order to kill everyone and take back the Infant, crying as it's not what he wants. The time he spent in the castle, along with killing comrades awoke his sleeping memories; he nearly fell to his death from Kagura's feather before she caught him.
Deciding to redeem himself by killing Naraku, Kohaku pretended to still be under his control. This was put to the test when Hakudōshi released a swarm of demon rats, and he was left to keep the shrine they came from open. Luckily, Kikyō destroyed the shrine, allowing him to continue his ruse. Later ordered to kill Parasitic Nymphs, Kohaku revealed he regained his memory to Kagura, gaining a shard of demonic energy to track the Fuyōheki with; Naraku's heart, the Infant had it in his possession, and if the Infant was slain, Naraku would die with it. Kohaku was then tasked with slaying demons, meeting a father and son who helped bind a wound he received while tracking a parasitic demon; it possessed the boy, who Kohaku chased after to free him from the demon. His sister arrived and drove the demon out, forcing him to leave to continue the ruse.
He eventually ran into Kagura again, and she tried killing him for his shard; Hakudōshi and the Infant had turned on Naraku and needed shards to gain power for their plan to replace him. When his determination showed Kagura the truth about their situations, she sent him away on a feather for his own safety. Sango followed, but a confrontation with Mōryōmaru forced him to reveal the truth to her. Kohaku then sought to find redemption for himself by traveling with Kikyō, and remains by her side until she tells him he must leave her to keep his shard pure. Saved from Byakuya by the timely arrival of Sesshōmaru, Kohaku forms a friendship with Rin and helps to protect her. Kohaku's jewel shard was removed but surprisingly, he did not die or wither away. This was due to Kikyō's light releasing the jewel shard and remaining in Kohaku's body, as explained by Kagome.
Despite not serving a combat role in the final battle with Naraku, Kohaku helps guard Rin after her rescue and gives Miroku his mask to protect him from the miasma.
Kohaku is a humble and kindhearted boy who cares for his family and is very close to his sister Sango. When Naraku controlled his mind he was forced to kill everybody including his own father, leaving him in a state of shock. Naraku then used the guilt-ridden Kohaku as a pawn and puppet for his dirty work. Though Kohaku was a gentle boy, he was easily manipulated by Naraku to commit terrible acts because of his weak heart. He desired to forget the memories of killing his father and companions and allowed Naraku to erase them so his mind would be more at ease. After several encounters with Sango and her comrades as Naraku's pawn however, he begins to remember his sister and his life as a demon slayer. The memories bring back his extreme guilt and sorrow, thus, he decides to kill Naraku and redeem himself. Kohaku is also very self-sacrificing about his own life compared to others, such as giving his own life to save Kagome, who had been taken hostage by Naraku. Kohaku's greatest desire is to face his father and comrades in the afterlife with an apology and be forgiven.