To be fair to Daphne, I have her pegged as flirty and happy to talk about perverted stuff, but still being basically a virgin and not being mentally ready to really go through with anything. I'm getting this mostly from the underwear questline, where she seems very aggressive/confident until you call her bluff, and then she quickly gets nervous and embarrassed. Teasing those reactions out of her was also really fun.
This does sort of make her sex scene feel a little weird, because she's on top. Up to that point, the more intense things got, the more Marcus starts to be in control. I guess you can write it off as her getting fed up/frustrated (which is brought up in the scene), but the end result is that it doesn't feel like there's anywhere else for her character to go anymore. Maybe that's fine, because we did just have sec with her so realistically how much more does she need to change?
I do however agree that Hermione feels really nondescript. Susan gets corrupted by putting ourselves in a position of authority and forcing her past her boundaries. Luna gets corrupted because she's already pretty perverse and we just tap into that and encourage it. Daphne gets corrupted when we keep calling her bluffs. Hermione... I dunno.