Lucifer
He pushes aside his work and cares for you, he makes sure you get enough sleep. He also cuddles you to make you feel warmer, the usual scary attitude he haves also goes away when you're sick, he would cheer you up by saying such stuff as "If you're sick, I'll be sad" or "I have so much I wanna do with you, so get well soon okay?" and all that cutesy stuff.
Mammon
"Tch, humans are so fragile. You're literally the biggest loser in this house" Says the personal punching bag of his brothers, he would be refusing at the beginning, but soon he'll just give up and took care of you- "You better be taking it easy, human! I know you don't like it when I tell you what to do, but I- I lo- lov-love you! And I want you back to your best self." He makes soup for you.
CR: Favim.com
Leviathan
He searched for a 'taking care of a sick human simulator game' and played it so he understands. After understanding it, he would bring a little game you both can play in bed, like cards, snakes and ladders, or ludo. He flips a fabric into a small rectangle and soaks it in lukewarm water, then he put it on your forehead. "Just like in-game!" He exclaimed.
Satan
Luckily he had already read a book on 'how to take care of a sick human' he's basically prepared for everything. He brought all of your favorite novels for you to read. While you read in his room, he cleans your room. Change your bedsheet to a new one, clean the dust, opens the window to let the fresh air go in, disinfect your stuff, get all of the dirty laundries out of the room, he also put a new air freshener of the scent that you like on the ac/fan. Once he had done cleaning, he went back to his room to find you sleeping peacefully, he doesn't wanna wake you up, but his room is full of dust, and you might get a runny nose. So he carefully brings you to your room.
Asmodeus
He would spend all of his money on the medicines there are in the drug store, he helps you to get out of the bed and takes care of you, helps you clean your body and stuff, also gonna help you moisturize then do your skincare. When you're sick, he likes to massage your head to calm your headache, sometimes shoulders or feet, to relax you.
Beelzebub
He would make pots of chicken soup, also feeds you a lot of veggies. He makes sure you're comfortable when you're bored of chicken soup he gives you vegetable soup instead, with a lot of garlic so you heal faster. He also buys you what you need and cuddles you a lot, along with kisses.
Belphegor
He gives you a bunch of pillows so you feel all fluffy, "You like to pat me and stuff, so I thought I'll pat you back" He kissed your head and cuddle you until you sleep, he's not the type to sing a lullaby but he's now doing it to you. "I love soft Belphie" You kiss his chin. He kisses you back on your forehead and rubs your back. He plays with your hair like the way you did to him.
Diavolo
He doesn't know how to take care of a sick person so just like every school nurse ever, he uses ice on your forehead. He entertains you by telling you his jokes that he got from his dad.
Barbatos
He would make everything for you, he also forces you to eat spicy food so all that snot up your nose gets the fuck out there. But after that super spicy food he makes, he gave you dolce as a treat. "Please eat this, Y/N. This is for your own sake" "HAAAA, THIS IS SPICIER THAN WASABI" It felt like it was your last meal every time he forces you to eat his super spicy food. You even starved one day, not wanting to eat his spicy food. But yes it did helps you, usually, your runny nose would stay up to 1-2 weeks. But this time it actually goes away after 5 days.
Simeon and Luke
They bake your favorite cake. They know you don't like the taste of medicine, therefore to cover the taste of the bitter medicine they crush it to dust and mix it with your favorite juice. Luke entertain you, you and him draw the night sky together. When you're about to go to sleep they hugged you and wish you to get well soon. So they can play again with you
Solomon
He made a formula to ease your sickness and entertains you with some funny magic spells that he find. "Trust me, this soup healed me every time I got sick. 3 hours after eating it I go back to being healthy" He cheekily said, oh with that innocent look on his face, you can't just deny it, you're not ungrateful, so you swallow the whole bowl without chewing it first. The bitter flavor is still in your throat even though you have chug a lot of water. But it's true, 4-5 hours after eating it your body feels a lot better. No coughing, or runny nose. So you thanked Solomon for it.