Demon Slayer Backstory's (Cannon)

Kaigaku

Word Count : 292

Kaigaku was an orphan that lived in a temple, a temple that was well known for its late-night demon attacks. If this sounds familiar that would make sense considering this was the same temple that the Stone Hashira Gyomei Himejima lived in before the demon attack that killed all the kids... all but two. The two that lived were the girl that stuck behind Gyomei and Kaigaku. Kaigaku was the child who let the demon into the temple. The reason is because hours before kaigaku was found by the demon, the children of the temple found out Kaigaku had been stealing the temple money, because of this the other children chased him out into the forest causing him to run into the demon. When he met the demon, out of fear he traded his life for the others living in the temple. 

After running away and reaching a safe place he continued to live in the streets until Jigoro Kuwajima found him and decided to make Kaigaku his disciple alongside Zenitsu Agatsuma. As he continued to train under Jigoro he became determined to become his successor and the new Thunder Hashira. Jigoro, however, insisted that both he and Zenitsu would become his successors, despite Zenitsu's weakness and timid nature.

Manga spoilers 

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During the Hashira Training Arc, Zenitsu receives word that Kaigaku had become a demon, and that Jigoro committed seppuku as atonement. Kaigaku had the unfortunate luck of encountering Kokushibo and, upon witnessing the Upper Rank's immense power, kneeled before him and surrendered in order to save his own skin. 

As a demon, Kaigaku had eaten many people and became powerful enough to become the new Upper-Rank Six of the Twelve Kizuki by the start of the Infinity Castle Arc.