*Giggles* Virgin sacrifice, I like that. Let's go with that for my guy. *Idea* He is brought in as virgin sacrifice, but due to his unique bloodline the demon that is summoned can get more power from him by winning his favor, or getting him to give up his body willingly. This would give him an excuse to stick around, but he could also hold it over the human's head that he doesn't "Have to" and can just take the minor power boost and return to his own realm any time he wants.

What cha think?