Kaigaku

Kaigaku is a former demon slayer who betrayed humanity and became the sixth Upper Rank in Muzan Kibutsuji's Twelve Demon Moons. He trained alongside Zenitsu Agatsuma under the former Thunder Pillar. He is a former demon slayer and former successor to the Breath of Thunder. He is the new Upper Rank Six, replacing Gyutaro and Daki. As a child, Kaigaku was an orphan and street child. He was taken in by Gemei Himejima, the future Stone Pillar, at his temple along with other orphans. He later stole the temple's money, was caught, and to save his own life, allowed a demon into the monastery, leading to the deaths of the other children. Even so, he became the senior disciple of Jigoro Kuwajima, the former Thunder Pillar. He despised Zenitsu for his cowardice and weakness, believing the old man was wasting his time on him. During one of his missions, Kaigaku encountered Upper Moon One, Kokushibo. A coward in the face of overwhelming odds, Kaigaku knelt and begged for his life. Kokushibo fed him Muzan's blood, transforming him into a demon. Upon learning of this betrayal, their teacher, Jigoro, committed ritual suicide without an assistant, condemning him to a long and painful death. As a human, he had mastered five of the six Thunder Breathing styles, but he was never able to master the basic, most important First Thunder and Lightning Style, which Zenitsu himself had learned.

He was sitting in the endless castle when Muzan suddenly gave him the order to go and kill several hunters the Fourth Upper Moon had captured. Kaigaku didn't know about the Pillars, so Muzan didn't think it necessary to tell him. Kaigaku, rushing through the corridors, was surprised.

"You?!"