I like the thought of the demons grooming (the cleaning kind) each other when resting. Especially Pierce, who would groom the others, organize the hairs around their horns and brush his fingers in their hair all the way through until he hits a knot and gently detangling it.
When he groomes Ava for the first time, she was very surprised by it. Ava doesn't have horns so Pierce just skips the first step and goes the detangling. When she asks Rhys about it he tells her it's a bonding activity among them and is pleased she's now part of it
Pierce also will do the thing where you pet your cat and you slide your hand all the way from the front to the back and slide along the tail all the way to the end with the other demons and vice versa
When he tried it on Ava he was disappointedly reminded that Ava doesn't have a tail. So she just ties a tie or a strip of fabric around her waist so that Pierce can happily pet as long as he likes
Pierce gets his own hair done by Asch usually and often tends to fall asleep on him as a result of how warm his hands are. Since Pierce is so big, Asch can't get him off so cuddle time only ends when Pierce says cuddle time ends.
Asch prefers Rhys the comb through his hair and in return he'll cuddle and pet him throughout the whole process. And he likes to give him lil kisses too. (Asch gives everybody kisses)
Noi likes it when Leif grooms his hair even though it's a little short. After Noi's hair is done they switch and Leif is the one getting his hair groomed. He always puts his head in Noi's lap during this so Noi always gives him head kisses. (Noi also gives every head kisses)
Edit: tried to put my feelings into words, not sure of it worked