I think mephisto is not a bad demon, (I like the demons so far-forgot the names though-), but I think they're using Rin just to make him strong or something.
Spoiler (mouse over to view)
Mephisto's brother said something like: I don't understand why papa and brother are so interested in him?
, maybe one has one motive and the other another. Meh,~ it's only the start but what do you think? ________________
Curiosity is a cockroach-like thing.