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He was originally a Demon Hunter and member of the Demon Killing Corps as well as Zenitsu Agatsuma's senior student when they were being taught by Jigoro Kuwajima. He later becomes Zenitsu's archenemy after his treachery.
Although prodigetic, Kaigaku is an arrogant opportunitist who follows strong and despises the weakness, especially when it obstructs something he wants, as seen with his bullying words against Zenitsu.
Deep down, Kaigaku is also a coward, who cowered in fear at Kokushibou and knelt in front of him, willingly accepting Muzan Kibutsuji's blood and became a demon as a result. Kaigaku also shows no remorse for his own treachery and cowardice, even teasing his master, Jigoro Kuwajima, as a worthless weakling just like Zenitsu after discovering the latter committed suicide out of shame.
Kaigaku was an orphan and a thief, who had to eat scraps and steal to survive. This established a certain belief in his head, he started to believe that strength was the only thing worth respecting, and those who can‘t see true strength or are too weak aren’t worth respect.
During his carrer as a Demon Slayer, Kaigaku was taken in by Jigoro Kuwajima, the Thunder Pillar. along with Zenitsu, and taught the Breath of Thunder style. Kaigaku became determined to become Jigoro Kuwajima’s successor and the new Thunder Pillar. However, Kuwajima revealed that Zenitsu would become his successor, since Zenitsu had potential to master the First Style in the Breath of Thunder, while Kaigaku had never mastered it even after mastering the other styles from the Second to the Sixth.
Believing he was losing his master's favor, Kaigaku began to despise Zenitsu for being merely a weak and timid man who earned his master's favor out of dumb luck. Thus, he started to bully Zenitsu constantly.
Later, Kaigaku went on a solo mission to hunt down Kokushibou, the Upper Moon One, but was overpowered. After the defeat, Kaigaku kneeled before Kokushibo and begged for his life. Kokushibo offered Muzan's blood to turn Kaigaku into a demon, which he accepted and became a man-eating savage. After finding out that his student betrayed him and renounced humanity, Kuwajima committed suicide out of shame.
Dimensional Infinite Fortress Arc
After the deaths of Daki and Gyutaro, Kaigaku was promoted to become Upper Moon Six of the Twelve Demon Moons and confronted Zenitsu as a rematch he always desired. Zenitsu informed his master's demise to his treachers classmate, accusing Kaigaku for the one to blame, but Kaigaku nonchalently disagreeded with him. He believed that those who praised him was good and those who don't was evil, and he didn't care about his master's death for ignoring him. Disgusted, Zenitsu called Kaigaku as a trash who can never master Breath of Thunder - First Style, and decided to part ways with his former prefect.
Since he was yet to master his Demon Skills fully, Kaigaku was ultimately beheaded by Zenitsu's Breath of Thunder Seventh Style: Flaming Thunder God. Kaigaku mockingly commented that their master do favor Zenitsu more for teaching him that style, until Zenitsu revealed that Flaming Thunder God was his own making, in order to fight alongside Kaigaku as equals, had him not turning into a Demon.
Hearing this, Kaigaku watched in shock and anger as Zenitsu was saved by Yushiro. Eventually, Kaigaku's head screams in anger as he disintegrated. Yushiro scornfully declared that Kaigaku will be given nothing since he never learned how to give, and Kaigaku's die-alone fate was sealed once he decided to let his desire inflate.
- It was widely speculated that Kaigaku was the very same treachous child in Chapter 135's flashback who betrayed Gyomei Himejima, who adopted him and eight other children, and lured a demon to his temple that killed other children out of fear of death, before framing Himejima for the murder.