Pleasure, power, help in fooling/convincing the others and probably a chance to figure out how to disrupt Red Eyes' magic in exchange for being unable to keep dong in pants, yeah, sounds like a bad deal. I mean, in the future, you will probably be able to screw everything for Ress (and for yourself), but helping or not is still a choice and most players will probably help Ress (cuz he's sexy and it's just common sense) on their first playthrough.
Well it is true that the demons powers can be very helpful and in the future save him in lots of tough situations but so far we are reponsible for:
-Making him fall Unconcious during a Important Mission harming his and his teams reputation
-Made him overly Horny and played with his desires or his friends desires
-Are probably responsible for why those thugs wanted to kidnap him in the first place
-Are probably going to cause some sort of panic or distrust if anyone learns Ress has a literal demon inside him he can not control or keep in check that can also manipulate people around him. For Ress and this whole Order are kinda... very Anti-Demons for Good Reasons.
So he might be less enthusiastic at the start then we may hope... we will probably be able to convince him though after some effort
We've been forcefully summoned into fitting vessel, i don't think we're responsible for unconsciousness and kidnapping, that is entirely Red Eyes' fault imho. We and Ress both are just unlucky.
Horny, yes, but we basically are just digging out buried feelings. Manipulative, but not really harmful in this particular case.
I'd love to say "we just gotta avoid overinfusing Ress and it's gonna be fine", but you got a point there. Although, we are really different from Red demons, so we may or may not have some time to convince Ress that we're helpful and he can't get rid of us anyway.
Not gonna be surprised if Ress will accept us after something like "best hot time in his life" with partner of our choice. XD
(P.S.: i'm probably overdefending my silly powerwolf joke, but ah well)