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So Pikachu line can be:
>From Kanto, where it is considered a "rare pokémon" but still managed to be introduced in Kalos and Alola, here developing surfing Psychic powers
>From Kalos, being later introduced in Kanto and Alola, here developing surfing Psychic powers
>From Alola having original Psychic powers and losing them when changing diet in Kalos and Kanto
In Kanto the Electric Gym Leader, who owns a Raichu, is actually foreign. Pikachu could have been introduced to Kanto during the war, recently enough to be considered a rare Pokémon. Surge's nationality can't tell us much though because military men travel all around the world.
In Kalos no one considers Pikachu rare. And the reveal trailer for gen 6 showed Pikachu emiting lightning from the Electric gym in Lumiose City, towards the rest of the world. Several trainers in that Gym have Pikachu and Raichu and Pikachu is part of the quiz of the gym.
I think Pikachu is Kalosian. And Kantonian trainers are obsessed with him because japs fucking love french stuff.