I actually rather like Mephisto's air. He's very demonic like the misfits. He's very free and seeks fun, and his greed (I don't have a better word to describe it) moves him. That type of demon, I feel, is more inclined to have positive reactions towards a human disguised as a demon. Because it's hella interesting to know a human disguised as a demon, especially since that human is in the higher ranks. :3
I like him because he's cute XD I dig his design a lot. I hope that the more he observes Iruma, the more his greed will push him towards Iruma so he can see what more he will do to change the Netherworld.
But the more easygoing he is around Iruma, the more I feel he's going to be on the pro-Demon Root/Primal/whatever it's called side.