In the Pokemon series, there’s a difference between male and female Pikachu – male Pikachu has a tail shaped like a lightning bolt, while female Pikachu has a heart shape tail.
Although male and female Eevee cannot be distinguished in past titles, things are going to change with Pokemon Let’s GO Pikachu/Eevee. If your partner Pokemon is a female Eevee, it will have a heart on its tail. This seems to only apply to the partner’s main Pokemon.
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