You are a trainer in the Pokemon Universe, except in this version of the Pokemon Universe, trainers become fused with their chosen partner Pokemon. You just so happen to be the partner of a shiny Nihilego; the parasitic jellyfish Pokemon. Your upper half is inside of it’s massive bulb, and you have control over it’s tendrils and its abilities

You (of course) live in Alola region, and you are the best friend of Lusamine