Note for GP: I acknowledge your message, but I have a reason on why that wouldn't matter much: The phantoms aren't inherently evil. Mist are pretty docile and just want a chat. It depends on many factors, such as the host's personality and the conditions around them. So no, no being is immune to becoming a phantom variant (Because in most cases, those variants would basically just be the same person with the same personality, only darker and more powerful to the point of being OP compared to their original form).
Can't have a true form without a phantom gem to use for that true form
