I like the idea that demons and angels don't need water to survive and most of the cast was deeply confused by MC drinking it so often because "it didn't taste good" and they couldn't wrap their heads around why MC was so "obsessed" with it.
And then they found out that water is crucial for a human's health and humans DIE if they go for a few days without it and suddenly a whole new fridge was installed in the kitchen filled with just water bottles, MC started getting "h2o deliveries" completely unprompted throughout the day, texts from all of them reminding them to hydrate, etc
"MC, here's your water."
“Oi, have you had yer life juice yet?”
"Remember to hydrate."
"I still can't believe you humans depend on this to survive. How stupid is that?"
Since demons don't have middle or last names, an angry MC resorts to using their titles when they're really mad.
"Did you seriously just turn the router off? After I've been refreshing this page all day? Lucifer, Avatar of Pride, turn it back on now."
"It seems somebody swapped my homework with blank pages. Mammon, Avatar of Greed, do you have anything to say about this?"
"It's 3am. Leviathan, Avatar of Envy, you need to turn that concert DVD off right now and go to bed or I'm snapping it in half."
"Satan, Avatar of Wrath. This is the third time today I've stubbed my toe on one of your cursed books."
"Asmodeus, Avatar of Lust. You're a doll. But it has been seven hours. I can't try on any more clothes. I'm exhausted."
"I know my name was on this cup of expensive ice cream. Beelzebub, Avatar of Gluttony, how are you going to make this up to me?"
"So help me, Belphegor, Avatar of Sloth, if you're late for class again, I'm the one who gets an earful of complaints. You're leaving this house right now."
Mc: don't you just hate the fact that you already graduated and finished school, have a good and decent job, but suddenly you got sucked into hell and forced to go to school with most of the subjects you never even learned in your FUCKING ENTIRE LIFE?!
Lucifer: mc, we said we're sorry
Mc: I AM NOT GOING TO JUST LET MYSELF GET YELLED BY YOU JUST BECAUSE I CAN'T GET GOOD FUCKING GRADE IN A SUBJECT CALLED "BASIC STANDARD MAGIC"
Belphegor and Satan watching from afar: *snickers*