Webback near the end of the hashira training arc, we see zenitsu be serious and later in the demon hide out place, we see that he trained so he can kill kaigaku.
Webas revealed in demon slayer chapter 144, titled successors, zenitsu’s letter brought him news about the sudden death of his master and former thunder.
Webas revealed in demon slayer chapter 144, titled successors, zenitsu’s letter brought him news about the sudden death of his master and former thunder hashira, jigoro.