That ghost/fairy pokémon that imitates Pikachu IS definitely my favorite design far, and has a spot on my team along with Popplio. It's so creative, and cute, reminds me of a voodoo doll.
And all those ideas of him taking the form of a dead Pokémon or the form of the Pokémon you caught it with are fun but let's be real, Gamefreak would never do that. It'll probably always be Pikachu the one doll they use, because it is the most 'loved' Pkmn on the franchise so it makes sense.
I definitely like the whole concept around it and I am hoping that, if it doesn't evolve, his ability/ unique typing are enough for it to be usable. I'm thinking it will end up like Klefki, and if so I'll be very happy and will use it
