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Extending even more my point, after he started working for Reigen, ONE made sure to write Serizawa in good moments. He made a friend, went to a New Years' Party, became the backbone of S&S and is depicted smiling many times. Even on the future omake, where Mob and Reigen are trying to deal with Mob's boss so he doesn't get fired, Serizawa is just... fishing. And he got his first catch and he's so happy about it. Compare this to Reigen being miserable for purposes of comedy every now and then.
I understand when people say to the ones who coddle adult male characters that they're "a grown ass man with dick and balls", but I don't think this applies that well to Serizawa Katsuya because ONE made this ex-terrorist a little too cute in my opinion.