I didn't even know that you planned a corruption system for the game but... ok, had to think of an idea on the fly here:
Since the game is going full VN route and not a mix of sandbox + VN such as Mythic Manor for an example, in the specific scenes in which a character's corruption will start or progress there could be two options (or three if you decide to give the players the option to not corrupt certain characters). Option one: MC uses his speech skills, if he has any, to slowly corrupt that character using whatever he knows about him/her, making that character submit willingly. Option two: MC uses his demonic powers (at least the ones the MC will have control of at that point) to force that character to submit, like a darker way of corrupting them which will also kind of corrupt the MC since he'll be accepting his demonic nature.
I don't know if I got it right, but my idea is this one.
