Who Is Kaigaku — The Demon Slayer Turned Demon?

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 SPOILER ALERT: This article contains major spoilers from the Demon Slayer manga, including shocking plot points, final battles, and major plot twists not yet covered in the anime. Proceed only if you're caught up or don't mind having key story elements revealed.

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While many fans remember Kaigaku only as Zenitsu’s enemy, Kaigaku's backstory and choices are deeply tied to pain, fear, and ambition. His betrayal of the Demon Slayers was not random. It was the result of years of struggle and inner weakness.

Kaigaku used to be a Demon Slayer himself, who trained under the same master as Zenitsu and even shared the Thunder Breathing style. But unlike Zenitsu, Kaigaku could never accept weakness — not in others, and especially not in himself. That mindset led him down a dark and tragic path.

ALSO READ: Why Tanjiro Kamado Never Became A Hashira In Demon Slayer?

Kaigaku’s Life Before Becoming A Demon

Before becoming a Demon, Kaigaku was just an orphan in Gyomei Himejima's orphanage. He grew up stealing and doing whatever it took to stay alive. He once encountered a Demon, with whom he made a deal and gave up the others in the orphanage. Eventually, he was taken in by Jigoro Kuwajima, the former Thunder Hashira.

Under his care, Kaigaku began training as a Demon Slayer and learned the Thunder Breathing forms. However, Kaigaku was never able to learn the First Form, which is the base of the entire style. This made him angry and bitter, especially when Zenitsu — someone he looked down on — became a student too. Kaigaku saw power as everything, and not being able to master all the forms made him feel like a failure.

His Fight With Zenitsu And The Final Lesson

Kaigaku’s final moment came when he faced Zenitsu, his old rival. By then, Kaigaku had become an Upper Moon and believed he was unbeatable. But Zenitsu had grown too. Not only had he improved his skill, but he created a new Thunder Breathing form, the Seventh Form: Honoikazuchi no Kami, on his own.

Their fight was more than just a battle — it was a clash of values. Kaigaku fought for power and pride, and Zenitsu fought for his master and the belief that even the weak can become strong. In the end, Kaigaku lost not only the fight, but also the chance to be remembered with honor. He died angry and bitter, while Zenitsu stood tall as the true heir to their master’s will.

The Reason Behind His Betrayal

Kaigaku’s true turning point came during a mission. He faced Kokushibo, the Upper Moon One, and instead of fighting or dying honorably, Kaigaku chose to submit. He begged Kokushibo to turn him into a Demon. This was when he officially betrayed the Demon Slayers. His reason was simple — he couldn’t accept death or weakness. He wanted power, even if it meant giving up his humanity.

In Kaigaku’s mind, the Demon Slayers had failed him, and he didn’t believe in teamwork or sacrifice. What he believed in was that strength was the only thing that mattered. By becoming a Demon, he finally felt like he could be strong without limits. But that decision showed his fear more than his strength.

Final Verdict: Kaigaku Was The Source Of Zenitsu's Strength

Kaigaku’s story in Demon Slayer is sad, but an inspiration for Zenitsu. He was a boy who wanted to be strong but never learned what true strength meant. Instead of facing his fears, he succumbed to them and betrayed everything he once stood for, which turned out to be an eye-opener for Zenitsu.

Kaigaku wasn’t evil from the start. He was just broken, angry, and scared. And in a world where monsters are real, sometimes the ones who lose to their own inner demons become the scariest of all.

Release Year	MAL Rating	Animation Studio	Genre	Watch On
April 2019	8.44	Ufotable	Action, Supernatural	Netflix

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