Nintendo has announced that starting on December 14th, 2018, players of Super Smash Bros. Ultimate that also have played Pokémon: Let's Go, Pikachu! / Pokémon: Let's Go, Eevee! and have a save file on their Nintendo Switch system, will unlock the Partner Pikachu or Partner Eevee spirits. They can also be unlocked by completing challenges too.

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Source: https://twitter.com/NintendoAUNZ/status/1069040555672383488