Kohaku

is Sango's younger brother. He is also a Yōkai Taijiya, like his sister and father, from the Village of Demon Slayers. He was manipulated by Naraku through most of the series, kept alive only by the Shikon Jewel shard in the back of his neck.

History
When he was still an infant, his mother died and his sister always took care of him along with their father (who later died).

During the story
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, 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. In chapter 529, 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.

Later life
In the last chapter, Kohaku is revealed to be traveling around with Kirara, slaying Yōkai for those in need. Tōtōsai gives him a new weapon, although he says it's heavier than his old one; Kohaku is thankful regardless. The old swordsmith then asks Kohaku if he still visits his sister Sango from time to time. With a smile, Kohaku laughs, saying "no, their home is kind of crowded."

Personality
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, it left 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.

Family
He is very close to his sister; though he was brainwashed by Naraku who resurrected him from death. Kohaku meets Sango and realized her as his older sister, though he hid himself from her. Once he regains his memory, Kohaku is determined to kill Naraku secretly without letting Sango know. He feels great guilt for causing Sango much emotional pain and believes he is unworthy of life. By The Final Act, he begins to feel the desire to live once more and redeem himself.
 * Sango

Enemy
Though he willingly obeyed Naraku for much of the series, Kohaku always despised him for manipulating him and causing him to kill his father and comrades and wound not only Sango but countless others. Over the course of time, Kohaku's memories return and he becomes determined to kill Naraku and avenge his loved ones killed by Naraku and also to redeem himself for being used as a killer. In the Final Act, Kohaku betrays Naraku and escapes with Kagura's help and from that point Naraku hunts Kohaku for his jewel shard. Once Kikyō uses Kohaku's shard as part of her plan to destroy Naraku, Kohaku becomes a major threat to his great enemy and Naraku is forced to plan carefully when dealing with him.
 * Naraku

Others
After she was abducted by Kagura, Rin met Kohaku in the shrine. They soon befriended each other. In fact, Rin lost her entire family who were slaughtered by thieves and Kohaku struggled to recover his memories when Naraku controlled him. At first their relationship went bad when Naraku ordered Kohaku to kill Rin in front of Inuyasha, Kagome and Sesshōmaru, but he refused to. They encounter each other a second time in a cave on Mount Hakurei. Kohaku protected Rin from the emerging demons, but also urged her to escape before she was killed. Later, he joined Sesshōmaru and Jaken after Kagura and Kikyō's death. In the later of the series, Kōhaku becomes protective of Rin, thus having their friendship continuing well.
 * Rin

His first meeting with Kikyō as an enemy, Kohaku was unsure of Kikyō until he fled from Naraku and refused staying with Inuyasha's group. He came across Kikyō and begged her to use his life to destroy Naraku, thus, redeeming him of his guilt and sins. He was not phased by her warnings her plan would lead to his death, he even accepted it gladly. During their journey together, Kohaku begins to have respect and concern for Kikyō, even worrying when she puts herself in danger. Kohaku mentions his main purpose for staying with Kikyō to Sango as, Kikyō's presence calms his spirit and he feels as if sins are cleansed and redeemed around her. When Kikyō dies, Kohaku is guilt-ridden he could not protect her and is resolved to carry out her mission to destroy Naraku with his shard.
 * Kikyō

Inuyasha has tried to protect Kohaku from death several times including a time with Sango and with Sesshōmaru. Inuyasha is determined to keep Kohaku alive for two reasons: he does not want Naraku to get what he wants (i.e. the Sacred Jewel shard in his back), the taking of which would lead Kohaku's death and also because he knows that Sango will be grief-stricken if her brother dies. Kohaku comes to respect Inuyasha greatly.
 * Inuyasha

Miroku, like Inuyasha, has desperately tried to save Kohaku several times during their meetings, even risking his own life in the process. Because he is in love with Sango, Miroku is aware what Kohaku's death would do to her and thus does everything in his power to protect his life. Kohaku later acknowledged the monk's bravery during his possession by Magastuhi as well as his sister's love for Miroku. As Naraku is destroyed in the Final Act, Kohaku asks Miroku to take care of his sister for him. By the Final Act, he clearly accepts Miroku.
 * Miroku

Kohaku joins Sesshōmaru after Kikyō's death and stays as part of his group a great part of the Final Act. Kohaku is respectful and admiring though wary of Sesshōmaru and spends much time guarding Rin when Sesshōmaru cannot. Kohaku forever is not afraid to beg Sesshōmaru to spare others' lives, an example being when his sister Sango nearly killed Rin trying to kill Naraku. Kohaku is the first to offer an excuse for her and desperately tries to convince Sesshōmaru not to kill Sango in anger. This shows that he very much cares for and would do anything to protect his sister, much like she did for him and is in essence a role reversal (i.e. Sango settling down as a housewife and her brother becoming more of the dominant protector)
 * Sesshōmaru

Kohaku gets along with Kagome very much, as Kagome risks her life to save Kohaku because she knows that Kohaku's death would be devastating for Sango, whom she cares deeply for. Kohaku says that Kagome reminds him of how Kikyō was, which meant that he always felt calm when he was around her. Kohaku also protects Kagome because she is best friends with Sango and gains respect when he sees how powerful Kagome can really be. There was a time when Kagome asked Sesshōmaru to spare Kohaku's life when he was going to kill him instead, that was when Kohaku was still under Naraku's spell.
 * Kagome

Trivia

 * The new Kusarigama that Kohaku received from Tōtōsai is extremely reminiscent of the corrupted weapon that he had when he was possessed by Magatsuhi.
 * As stated by Sesshōmaru's mother, Kohaku can't be revived by Tenseiga because his life has already been unnaturally prolonged.
 * In the early episodes of the anime, the jewel shard that kept Kohaku alive was in the middle of his back. Towards the end, it was at the top of the neck.