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We only really need a few Pokémon as Fighters. The others are better as Poké Ball Pokémon.
Pikachu, Jigglypuff and Mewtwo are fine.
Maybe Pokémon Trainer too to represent the concept of a Pokémon Trainer, and maybe Pichu just to represent Baby Pokémon. But that last one feels a bit much.
All the other Pokémon Fighters and having more besides those feel like overdoing it.
Also, not every gen needs a "rep", rather the Fighters of a series are there representing their series. The few I listed are good enough for Pokémon.
