FTR, I don't consider gym leaders as main characters per se. When I say that, I mean characters like Red, Green/Blue. Or whoever is seen for more than 10 episodes worth. Yes, gym leaders are important, but you only see them once or twice, or on special occasions like Brock. But you should have known what I was talking about when I said main.
Besides size and design difference, whats the point of evolving a pikachu? New moves I guess? I'd imagine by this point it's defense and hp won't change much even if I did.
I seem to recall one game where one of my pikachus seemed to have had a decent defense. Did they change that, or does it depend on the stats? I read that even if you caught 1 kind of pokemon, catching the same kind & same gender, but could differ on stats and personality trait.