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. But recently, Kohaku has somehow got his memory back and still takes orders from Naraku to get to Naraku's heart. Kohaku, at first before being slain, was inexperienced with his weapon, a kusarigama (a sharp sickle on a chain). He then becomes a rather advanced wielder with the power of the Shikon Jewel shard. he met his sister for sometimes at first he cannot remember her and tried to kill Kagome under Naraku's mind.

After Kagura dies, he hopes that he can 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 539, 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.

Later life
In the last chapter, Kohaku goes with Kirara and a new demon-slaying weapon which Tōtōsai made for him, so he leaves and becomes a Yōkai Taijiya that helps people. When Tōtōsai asks him if he will visit his sister Sango, he replies that he will not, since the house is getting too crowded.

Personality
Kohaku is a humble and kind-hearted 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 slight state of shock. Early on Naraku used Kohaku not wanting to remember as a way to keep control over him. However, when his memories returned on their own, he did not seemed to be affected as severely as before.

Sango
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 his sins and believes he is unworthy of life. By the Final Act, he begins to feel the desire to live once more and redeem himself. He still clearly loves Sango and gives her care and safety to Miroku.

Rin
After 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.

Kikyō
Their relationship started out bad. Kohaku slayed Kochō and Asuka by mistake after recovering his memories. After Kagura's death, he joined Kikyo, whom protected him and his shard, keeping it pure and away from Naraku until she was killed.

Inuyasha
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 neck), the taking of which would lead Kohaku's death and also because he knows that Sango will be grief-stricken if her brother dies.

Miroku
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 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 acknowleged the monk's bravery during his possesion by Magastuhi as well as his sister's love for Miroku. As Naraku is destroyed in the Final Act, Kohaku begs Miroku to take care of his sister. By the Final Act, he clearly accepts Miroku.

Quotes

 * "I want to remember... that girl... myself... and everything. But..."
 * "What's this? This feeling... it's... scary... Like there's something I shouldn't remember...!"
 * "Sister!"

Trivia

 * The new Kusarigama that Kohaku received from Totosai is extremely reminiscent of the corrupted weapon that he had as Magatsuhi's body.
 * As stated by Sesshōmaru's mother, Kohaku can't be revived by Tenseiga because his life has already been unnaturally prolonged.
 * Akiko Yajima voiced Kōhaku in the Japanese version, and she also voiced Yura.
 * 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.