Kohaku

Kohaku is Sango's brother, whom was killed by a demon posing to be a castle's lord at one of the Demon Slayers's missions, who made him kill every Demon Slayer that was there. He is later revived by Naraku as a human puppet and has a Shikon No Tama jewel 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.