I actually don't know where people get the idea that the soul (You) is metaphorical and Kris is just giving in to dark impulses or something. Many characters both vaguely and specifically refer to Kris's soul, specify that the human soul is shaped like a heart, the bloodstained wagon and bird cage in Kris's room implying they're not unfamiliar with being possessed or having a wandering soul. Noelle being able to tell that Kris is undergoing dual identities right now and many people around Kris being able to tell they're not behaving like they usually do.