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Pokemon characters aren't written well enough to have any sort of devout fanbase unless someone really wants to fuck them, or it's a fujo ship. Pokemon characters have very very VERY little going on in the writing department and the best you can get out of most of them is "I liked their character arc". most fan works go off of the artists' personal interpretation or make silly jokes and run with it, I'd like to point to SubMas and how those fuckers have been churning out art for over a decade now when Ingo and Emmet have LITERALLY no character. Even the Pokemon anime is so dedicated to it's monster of the week formula that there are barely any worthwhile character arcs and said arcs are somehow even less substantial than the games.
I say all of that to say that Pokemon characters aren't "underrated", they're rated perfectly fine in that they're only talked about sometimes. I will admit that Gen 9 has done a lot more with each character than pretty much every other generation, and has a very good cast as a result, but even then they're still on the tier of like....a 90's JRPG, meanwhile there are games with significantly more fleshed out stories with much more interesting cast members that people would rather get invested in--Pokemon's character writing can only appeal to you if you've never played a game with good story before.