I would like for her to go to her Native American grandfather for help. This is more for the sake of addressing the lack of POC protagonists in YA fiction than a desire specifically to write a Native American, but considering the North American setting, this seems like the best fit. (For the supernatural being, not the main character. I recognize that there are many Americans who are neither Native American nor white.)
My question: are there any Native American legends involving either shapeshifters impersonating human beings, or spirits possessing or otherwise controlling human bodies? If so, which tribes should I be looking at? If not, are there any other non-European traditions that an elderly man who is reasonably connected to his culture would be able to share with his granddaughter (who, like many adolescents, was not particularly interested in her heritage until she needed it.) I intend to fully research the culture once I know what it is, but I really need a starting point.
The setting is modern day United States, specific region to be determined later. Or possibly Canada.
The search: I googled "Native American shapeshifters" (and only got information about the skinwalker, which is not really what I'm looking for), checked "Native American mythology" on wikipedia and followed the links to specific tribes (but not all of them, because I got frustrated), searched Myth Beasts, searched Giants, Monsters, and Dragons: an Encyclopedia of Folklore, Legend, and Myth, read the wikipedia pages on spirit possession and demonic possession (although I definitely don't want this to be about demons in the Biblical tradition), and browsed this community's tags for native americans, religion: native american, folklore, and demons.
Any assistance would be greatly appreciated, even if the answer is that what I'm looking for doesn't exist. I don't mind coming up with my own creature, but I'd like to work with established mythology if possible.