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