Weeeeelll, not to nitpick, but the soldier is lost in a make-believe world, and is just beginning his association with a certain scantily-clad demon. Who knows where it would go given a week or so? Same thing with the boy, we're given to understand that he's been possessed for just a couple of days, coinciding with the beasties attacking a certain village.
In terms of long-term possessed/visited/franchised NPCs, there are two who have been able to go long-term without physically deforming or going bat-guano crazy. In fact, there are very specific dialogue trees dealing with this phenomenon. "Aren't abomination usually insane horrors?" and "If you keep your humanity, then you can't be a real abomination," etc.
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Of course, there's a certain kitty-cat who's been possessed for Gah-knows-how-long, and it still looks like a kitty. Kind of throws a wrench in the long-term-possession-makes-you-deformed theory.
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