Summary

Zenitsu transforms after learning of mentor's brutal death, preparing for intense battle with his old bully.
Kuwajima's seppuku unfolds in a mysterious letter, pushing Agatsuma to seek revenge against Kaigaku.
Zenitsu's upcoming brutal battle with Kaigaku will shape the war against Muzan, promising far-reaching consequences.

Demon Slayer's Hashira Training arc is focused on the events that unfolded after Nezuko conquered the sun and two Upper Moons were killed in the Swordsmith Village. With immortality closer than ever before for Demon King Muzan Kibutsuji, the Demon Slayer Corps prepare for an existential battle that could very well define the future of humanity.

The arc focused on several pivotal events, including the Hashira meeting led by Amane Ubuyashiki, Gyomei's backstory, the rift between the Shinazugawa brothers, and the relentless pursuit of the Demon Slayer Mark. But one small scene that was perhaps overlooked by most fans will have a far greater influence on future events than everything else so far. It will not only transform one of the demon slayers but also influence an intense battle in the Demon Slayer: Infinity Castle trilogy in ways no one can predict.

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By Mirza Aaqib Beg

Zenitsu and the Mysterious Letter

Agatsuma Learns the Horrifying Fate of His Mentor

Anime: Episode 62, titled "Stone Hashira Gyomei Himejima"

Ever since Zenitsu Agatsuma made his debut, he has been awfully cowardly and annoying. While he does improve dramatically as a fighter, his heroics remain the result of his subconscious programming and relentless training done under the watchful eyes of Jigoro Kuwajima. Even in the Hashira Training arc, Zenitsu causes unnecessary nuisance and complains constantly about the harsh training.

Zenitsu, perfect it! It's alright to cry or go off by yourself. Just don't ever give up. Believe in yourself. You endured all that hellish training. You'll be rewarded for that without question. Work to forge yourself like a tempered blade... And become the most resilient of all! Focus on what you do best! ~ Jigoro Kuwajima to Zenitsu Agatsuma

Everything changes when he receives a mysterious letter while training under Stone Hashira Gyomei Himejima. At the time, the scene does not appear that serious. In fact, Zenitsu is busy with his usual antics, which hides the sinister nature of the message he is about to receive.

Through the mysterious letter, Agatsuma is informed that his mentor and former Thunder Hashira, Jigoro Kuwajima, committed seppuku. As if this was already not horrible enough, Zenitsu also learns that the process was long and painful, as it proved to be too hard to cut his mentor's head off. In his last moments, Kuwajima endured unimaginable pain and suffering that he should have been put through.

The Butterfly Effect of the Letter Addressed to Zenitsu

Zenitsu Transformed As a Fighter Following Jigoro's Death

After Zenitsu learned about Kuwajima's passing, he was understandably heartbroken. But instead of giving in to despair, he had an unexpected transformation that surprised even his close friends. Agatsuma stopped complaining about training or talking to anyone. He instead isolated himself and focused on the objective that he had set for himself.

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By Mirza Aaqib Beg

Interestingly, the letter also mentioned that Kuwajima committed seppuku after learning that one of his disciples, named Kaigaku, had become a demon. This turned out to be an unbearable burden for him, and the former Thunder Hashira felt that death was preferable to living with the shameful betrayal. Kaigaku went on to replace Gyutaro and Daki as the new Upper Moon Six.

Many years ago, when Zenitsu was training under Jigoro, Kaigaku used to ridicule him. So, when he realized that the same bully was responsible for the death of the only person who believed in him, Agatsuma was understandably appalled. Unlike the bullying, Zenitsu knew that he could not forgive Kaigaku this time and vowed to exact revenge.

Until you die, you haven't lost. You may rub your face in the dirt... You may lose your home or drink muddy water... People may curse you for stealing money... But as long as you live on... You can win someday. You will win. I have moved forward with that belief. ~ Kaigaku

In the Demon Slayer: Infinity Castle trilogy, the focus will turn to an all-out war between the Demon Slayer Corps and Muzan's underlings. While most Hashira and other demon slayers will fight in groups, Zenitsu will confront his arch-enemy Kaigaku alone. Driven by a feeling of revenge and sorrow, Agatsuma will push his old bully to look for any sense of regret for his actions.

But Kaigaku, who has lived a life driven by self-preservation, will refuse to shed a single tear for his old mentor who guided him when he needed it the most. Jigoro's brutal death and the unremorseful justification will push Zenitsu beyond his tolerance, and a brutal battle will ensue. While most fans are excited about the fights involving Akaza, Doma, and Kokushibo in the Demon Slayer: Infinity Castle trilogy, Agatsuma's battle with Kaigaku will be just as exciting. It will shape the war with Muzan Kibutsuji and have far-reaching consequences.

Demon Slayer: Infinity Castle trilogy's first installment is scheduled to be released in the United States on September 12, 2025.