Interesting thought. I can't answer the question theologically; it's speculative. In terms of psychological reality, yeah, I think so. People's awareness only covers a limited part of their actual personhood at any one time. Jung used to talk about consciousness (of any single person, in this case) being like islands poking above the ocean. The consciousness shows up here and there, and is different in each place. Some places you'd recognize easily, others not so easily, though they're all still part of the same person.
And that's without talking about the ocean or the parts underwater.
Some folks, according to Harry Stack Sullivan, undergo a "malevolent transformation" and identify with actual evil. One doesn't meet these folks too often outside of psychiatric hospitals or prisons, but they're out there. They will sometimes identify with the demonic, which is an Archetype in the Platonic sense. One can certainly be possessed by Archetypes, such as The Old Wise Man or The Crone or The Great Mother or Venus. The Demonic is one of those, and it can very easily possess people and do not very nice things with them. Alastair Crowley, if I have his name spelled correctly, was one of them. Charles Manson would likely have been another.
When you get a lot of Archetypal stuff going on, reports of what happens in those places seem to become unreliable. Whether that reflects an actual weirdness in reality or in people's reports about reality in those situations, I don't know.
I think that it's much simpler to dismiss reports of the demonic, of course, if you've never actually seen or experienced something directed personally at you with what amounts to a soul destroying intention. If you don't believe in soul, and many fine people do not, please feel free to substitute a word that you feel has a parallel and equivalent if non-theological meaning. I would suggest happy thoughts and joyful pastimes are a much more fruitful way of dealing with life.
And for goodness' sake, why are you so curious about demons and possession when you could be just as interested in passionate fascinations and thrilling accomplishments? They're actually probably more fun to achieve and considerably more meaningful in the long run. In my opinion.