Of all the demons to get into human realm, mephisto may be the least adapting. His life is about understanding custom, but of demon perspective. It will be like Jack Skellington in Christmas Town, overwhelmed and missing a key to understanding.
Mmm, could Mephisto be the reason Sullivan made Iruma chant a forbidden spell on the first day of school? also I think is super cool that Mephisto doesnt follow the rules because he is inspired on a demon of literature, not one found in the Ars Goetia or myths like the majority of demons shown so far