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Demon Slayer has many memorable faces, but Kaigaku slips through the cracks until it is too late. He is not someone new when he is introduced as one of Muzan’s Twelve Kizuki. He was always there, hiding in plain sight and stitched into the story from Gyomei to Zenitsu without anyone realising how much damage he would eventually bring.
Kaigaku’s Dark Past With Gyomei Himejima
Kaigaku started as one of the orphans under Gyomei’s care, a temple child who did not care much for rules or guidance. While the other kids respected Gyomei and their home, Kaigaku was the one caught stealing. When punished, he stormed off, and that single act spiraled into tragedy.
He encountered a demon while he was out in the night and out of desperation, he led it to the temple and helped it slip inside by extinguishing the incense that kept demons away. As a result, seven children were devoured that night. Gyomei managed to protect one, but by morning the demon had turned to ash, and the child, still traumatised, could not explain what had happened.
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Gyomei, with nothing to prove his innocence, was blamed for the deaths and got sentenced to execution. However, his life was only spared thanks to intervention from Ubuyashiki.
Kaigaku’s Training Under Jigoro Kuwajima
After the temple incident, Kaigaku moved on and found a new place under Jigoro Kuwajima, former Thunder Hashira and Zenitsu’s master. He trained under Jigoro with full dedication, wanting to master Thunder Breathing and expected to become his only successor.
But, things shifted when Jigoro adopted Zenitsu, a cowardly boy full of fear but with an unexpected spark. While Zenitsu doubted himself constantly, Jigoro saw something in him and never gave up on that belief. He trained both boys harshly but cared deeply for them.
This balance was shattered when Jigoro announced that both Zenitsu and Kaigaku would succeed him. Kaigaku took this as betrayal, as he could not accept being placed on equal ground with someone he considered weak. Eventually, pride clouded his judgment, and his hatred for Zenitsu grew. Years later, Kaigaku ended up face-to-face with Kokushibo, the most powerful demon under Muzan.
Instead of fighting till the end, Kaigaku gave up by accepting demon blood and turned into what he was supposed to fight against. He climbed the ranks quickly, eventually becoming Upper Rank Six, replacing Gyutaro and Daki.
Jigoro’s Death and Kaigaku’s Ultimate Betrayal
The news of Kaigaku’s transformation reached Jigoro, and it shattered him. He had trained Kaigaku to protect people and not to consume them. Unfortunately, Jigoro, unable to carry the weight of this shame, took his own life without witnesses and without explanation.
Toward the end of season four, Zenitsu reads a letter whose contents are hidden from the viewers. But that letter, from Kaigaku’s betrayal to Jigoro’s death, explained everything. It flipped a switch in Zenitsu, and the same boy who once needed sleep to fight was now fully awake, silent, and deadly. A battle between Kaigaku and Zenitsu is now inevitable, and it won’t be a normal fight.
The upcoming Infinity Castle arc, which is already making waves in Japan, will bring many brutal showdowns between the Demon Slayers and the Twelve Kizuki, per Game Rant. Kaigaku’s fight won’t be about rank or power alone; it will be personal. It will be between one student who betrayed everything and the other who never stopped believing.
ZENITSU VS KAIGAKU pic.twitter.com/ltjhTg4eI8
— Demon Slayer Daily (@DemonSlayerSc) June 28, 2025
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