So there is this player in my game who has a character with some inhuman abilities tied into a spirit from the Nevernever sharing his brain space (kind of a Harry-Lasciel sort of thing). Now, the player wants his powers (inhuman strength, toughness, recovery) limited by the spirit's nature/agenda. The spirit is a spirit of justice, so anything involving vengeance or even mercy can conflict with the spirit's nature, negating or weakening his powers.
How would you guys handle this in terms of game mechanics? We've played with some ideas before but it hasn't really come up yet in the campaign. We were thinking of something based off of the vampire's hunger stress track or maybe something like a sponsor debt. Or should it be some sort of catch?