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So I wonder if Pikachu is female or male, because in the Movie Pokémon - I choose you!, Pikachu was voiced by a woman. If he was male, I don't think they would voice that Part by a woman.
Also, Ash’s Pikachu has no heart shape at the end of it’s tail, but that proves nothing, because that wasn’t a Thing when Pomémon was created.
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Ash's Pikachu is male. This is confirmed in the anime episode Where No Togepi Has Gone Before!, which aired after Diamond/Pearl introduced heart-shaped tails for female Pikachu. In that episode, a Togepi uses Attract on both Pikachu and Meowth, and it works on both occasions. Since Meowth is known to be male, and Attract only works on Pokémon of the opposite gender, that means the Togepi must be female, and Ash's Pikachu must also be male.
The Pikachu in Pokémon - I Choose You! is explicitly the same Pikachu as in the main anime, as it takes place in a parallel timeline that splits off from the original series' timeline. Therefore, the Pikachu in I Choose You! is also male.
Your main argument seems to be that Pikachu is voiced by a woman, and must therefore be female. This is not the case. It is extremely common for young male characters in anime/cartoons to be voiced by women. For example: Ash Ketchum has also been voiced by a woman - in both Japanese and English - for the last 22 years.
