Aha, I knew it was that hotel ghost! It's weird though that she was able to force Karen out of her body so easily though, and because of cosmetic removal cleanser? I'm also surprised that Nakamichi didn't notice the eye color change as an early indicator that she was being hijacked.
edit: So something else just occurred to me...Why is Nakamichi keeping his meeting with greenhead a secret? If Karen knew of certain implications to her existence, wouldn't it help her to be more cautious and aware of potential dangers? Oh wait, what am I talking about? Gotta have that artificial tension supplier whenever the plot demands it.😷