Why Kaigaku will change Zenitsu's entire character arc in Demon Slayer Infinity Castle, explained

With the Demon Slayer Infinity Castle movie already creating a worldwide sensation among anime fans, one of the key highlighted battles will change a beloved character completely. The battle in question here is between the former students of Jigoro Kuwajima- Zenitsu and Kaigaku.

While many fans may go into it with a normal demon vs Slayer expectation, the battle is much more than that. The sole reason for the high intensity of the battle is strictly because we are introduced to a completely different Zenitsu, a much more mature and stoic person, rather than the usual cowardly and scared boy.

Disclaimer: This article expresses the author's opinions and contains potential spoilers from the first Demon Slayer Infinity Castle movie.

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Kaigaku's introduction changes Zenitsu completely in Demon Slayer

The battles in the Infinity Castle movies have a personal touch, since the Demon Slayer series' encounters were all simple encounters between the Slayers and the demons until this point. Every conflict has a backstory and is, in one way or another, an act of retaliation or avenging others.

In the first movie, the series introduces the character of Kaigaku, an acquaintance from Zenitsu's past, who has taken over the role of the Upper Moon Six demon after Gyutaro and Daki's death in the Entertainment District arc. While the Infinity Castle was their only full-fledged confrontation, their narrative runs deeper, and what partially led to Zenitsu's character change.

As per the series' narrative, Kaigaku and Zenitsu both trained under the former Thunder Hashira, Jigoro Kuwajima, before joining the Demon Slayer Corps. However, despite both being trainees at the same place, the relationship between them was filled with bumps. While Zenitsu always respected Kaigaku, the latter had no ounce of respect for the former and often mocked him.

The fans might have noticed that Zenitsu's character change occurred right after he received a letter in the Hashira Training arc. So what was in that letter that changed Zenitsu's personality before entering and inside the Infinity Castle? The letter informed Zenitsu about the shocking death of his former master and grandfather, Jigoro. Furthermore, the reason for Jigoro's suicide was Kaigaku.

Kaigaku had betrayed the Demon Slayer Corps and joined the demons after being turned by the Upper Rank One demon Kokushibo. Upon learning about the betrayal, Jigoro, being a former Hashira, was heartbroken and committed seppuku in atonement for Kaigaku's actions. Furthermore, when Jigoro died, there was no one to behead him and give him comfort, making it a slow and painful death.

For Zenitsu, it was all about avenging his grandfather and punishing Kaigaku for his betrayal. The moment he entered the castle, Zenitsu had his senses set on Kaigaku. After a back-and-forth between the two, Zenitsu defeated Kaigaku with his own invented form of Thunder Breathing, the Seventh Form: Flaming Thunder God.

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Final thoughts

A serious and stoic Zenitsu as seen in the movie (Image via Ufotable)

The battle between Kaigaku and Zenitsu in the first Demon Slayer Infinity Castle movie marks a turning point in Zenitsu's character arc in the series. The fans have never seen this version of Zenitsu before, and it was actually a brilliant choice by the mangaka to appropriately portray the gravity of their battle.

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