I got a lot of flak a few months ago when I said Yamato's role in the story was to enable other characters' stories.
We need someone to have Oden's journal.
We need someone to protect Momonosuke.
We need someone to stall Kaido once Luffy is thrown outside.
We need someone to teach Momo how to be a dragon.
We need someone to be Momo's mentor figure while Luffy is busy.So, someone was made to fill those blanks. How do you make people interested in such a character? Create big expectations about him.
I do see Yamato having a character arc, it's just does not seem to be the one people want. If it's meant to make him fond of Momonosuke and realise his role is as Wano's "Guardian Spirit", every choice so far makes sense. He starts in Point A ("I want to go to the sea") and ends in Point B ("I see this is my place after all"). Wants vs Needs. If not, then it's questionable while he got those roles in the first place. His character arc should have been something else.
You know, given his background (20 years of isolation and suffering, with pretending to be another person being the only thing to keep him sane) I thought Yamato would be a little too naive, or a little too nutty, or a little too sheltered or all of the above. And he was like that in the first few chapters, but then as soon as he met Franky he was basically the most mature person in the entire Onigashima already and then he went on to become Momo's mentor. Even the Oden thing was kinda dropped.
It all felt a little artificial, I don't know how to say it