Depending on the character in questions personality, it might be possible to feed his APP or material possessions into it in some way - so if he suffers some disfigurement or even has his car/clothes/stuff negatively affected in some way.
I like the idea of using alleigence - may 1 point for noticing he is the best dressed person at a party, 3 points for driving someone to tears and feeling good about it due to pointing out their percieved inferiority etc.
Maybe consider insanities? I imagine there is a lot of potential for fun if the same character also had, for example, Quixotism! (he kills a goblin, but feels like he slew a Balrog!)
I think a lot would also depend on the relationship between the monster and the character - are they aware of each other? Can they communicate? (If so I imagine the monster would drive him to pick on fairly weak targets - I'm guessing it doesn't care how the guys ego is stroked, merely that it is and fairly regularly).
This is a fantastic idea for roleplay though - loving it!