Pisses me off when I'm reading obey me headcanons and they're just so blatantly oversimplified and the author adds nothing to the characters. I'm not a writer myself, but man do some of those headcanons feel lazy and repetitive.
Lucifer : sadistic bastard who's mean and or tired and stressed 24/7.
Mammon : gets flustered and tsundere VERY easily.
Levi : mammon 2.0 but more extreme and self depricating to the max
Satan : gets angry very easily and when pissed destroys the school and the whole devildom population if not held back
Asmo : lust and cutesy bestie who you have a spa day with
Beel : hungry and far too nice. A dog practically
Belphie : sleeps and sleeps. Too tired to do anything
Diavolo : happy to be there, Laughs a lot, dog 2.0
Barbatos : the tea overlord who provides you with high quality tea everytime you're in the vicinity
(can't recall anything for the rest atm)