Be cute and die

Each of the brothers and side characters is totally different looking in their demon form so I’m assuming they’re all different kinds of demons which means field day for Mc

Mc collects scales and feathers and whatever the hell they shed and collects them like some people collect rocks

They like them so much they keep them in a special place, and sometimes wants to eat them (I want to eat the pretty pink rocks) (is that weird? idk they just look yummy)

Lucifer molts. Mammon probably tried his best to get ahold of some of these feathers to sell but fails every time. All Mc needs to do to get one is ask. Mammon nearly combusts but does not take it since it makes them oddly happy

Mc must ask the craziest questions like "Do you ever polish your horns?" or "Do you wag your tail when you're happy?"

it appalls everyone every time but Mc doesn't ever get the memo but they're all too nice to tell them you don't ask those things

They probably had books on demons in general and maybe the brothers in specific. When Satan asked to see them, he was furious that he has to share a book with Lucifer since humans think they're the same person. After every thing was over, he had to buy them a new copy. You can guess what happened

The demons with tails are the embodiment of "Don't touch my tail" except Mc ignores this rule and they let them. Levi, Belphie, Satan, and Barbatos are not safe and they know it

The demons with wings know Mc loves admiring them and they are all too whipped to tell them no when they ask them to take them flying

Mc's all time favorite thing is when the demons shift while asleep so they can just admire them in their natural habitat. They treat it like a nature documentary and narrate it even after they wake up. more than once everyone except Barbatos and Belphie has been jumpscared upon waking up to find Mc about an inch from their face narrating their exact actions