I don't think those are different forms -- if you're talking about Vulca, she's a different individual from me. We have theories on why we look so similar, but nothing certain. I do have a few different forms I can take, but none are full Pokémon, and I can certainly possess Pokémon... And you could potentially consider me to be Pokémon-adjacent.

I myself am primarily a spirit of winter. I represent certain aspects of Winter. Over my lifespan I've also incorporated aspects of Winter-Predators - that is, predatory animals that can remain active in wintertime, mostly warm-blooded birds and mammals - and Protection, and as such I could be considered a spirit of either of those as well.

As for what it means to be a spirit... I'm from another world, known in your language as the Shadow, which runs parallel to the material world and has very different physical laws. It's a world driven by perception and interpretation. Whenever something has sufficient emotional weight to it in the material world, it creates a spirit double of itself in the Shadow.

Beings from the Shadow do not interact very well with the Material. Unless I'm materialized, or anchored to something or someone, I'd rapidly lose energy just being here, and materializing, itself, takes meaningful effort as well. ... still, worth it.

There's some who theorize that Pokémon themselves, and perhaps other similarly supernatural beings, are a result of long-past unions of mortal life-forms and spirits from the Shadow, like myself. I don't know if it's true. I do know that your Legendaries often have particular influence over spirits on the Shadow side.