If you’ve been following the Hashira Training arc, you likely noticed a change in Zenitsu. The letter that changed his behavior contains a tragic revelation. The anime may not have unveiled it yet, but fortunately, the manga gives us a glimpse. Be warned: there are manga spoilers ahead.
What was in Zenitsu’s letter in Demon Slayer?
Demon Slayer Chapter 144 confirms what was said in Zenitsu’s letter as he speaks to Kaigaku in a heated discussion. The message indicates that Jigoro Kuwajima, Zenitsu’s grandfather (not blood-related), took his own life after his former apprentice, Kaigaku, turned into a demon.
Both Zenitsu and Kaigaku were trained by Kuwajima, who previously held the title of Thunder Hashira. Kaigaku then became a Demon Slayer, just like his comrade. Yet, instead of heading down a righteous path, he eventually betrays his loved ones by leading a demon into their home to save his own skin. This was also revealed during the Season 4 episode, depicting the Stone Hashira’s backstory. Sometime after this, Kaigaku gives into the temptation of unlimited power and takes up the mantle of an Upper Rank Six demon.
Due to this betrayal, Jigoro’s guilt consumes him, as he believes he is responsible for creating such a demon. Even worse, he was all alone when the seppuku act was performed, which requires another person traditionally. But since there was no one else to finish off Kuwajima’s death, he suffered for quite a while until he passed on.
Once Zenitsu learns of his beloved master’s fate, he becomes almost emotionless, which is why his demeanor changes in Season 4, Episode 7. Fortunately, it shouldn’t be long until the anime also covers the letter, considering that the reveal is only a few chapters away from where the show is.
