Mephisto from Blue Exorcist seemed to fit to this one because personally, for me, he's a suspicious character. I mean, he really said that he's naturally evil because he's a demon, but apparently his love for humans surpasses his nature. His plans were still vague—I mean, is his fondness of humans really the only reason why he turned against his father? Or are there other reasons? Also, I think he could easily turn sides if he wanted to; after all, he's still a demon, and demons tend to be tricky and really unpredictable.