How would Toriel, Mettaton, Sans, and Flowey play with their human friends hair? Let’s say they have short hair cause we don’t see it as often :)
She adores messing with your hair. Brushing it, combing it, helping you style it--she's a mom through and through. And, bonus points: her claws feel so nice when she runs through your hair. It's like a little scalp massage. You could probably fall asleep when she does that, honestly. (Which you have, multiple times.)
If there's one thing Mettaton loves, it's getting to help people with make-overs--you included. You might not have a lot to work with, having short hair, but he can make do! He's going to style the heck out of your hair. Darling, trust him, he'll have you looking like a movie-star by the end of it!
He tends to do it without realizing it, just gently running his phalanges through the short strands while the two of you lean against each other. It's very relaxing and comforting, which is pretty much what your whole friendship with Sans is like. He just has a way of making you feel safe.
The feeling of...roots? Vines? Whatever he has, running through your hair is a little strange, but also very sweet. The fact that he even does it is enough to bring a smile to your face--just don't let him see it, or he'll get defensive and pout. (And mess up your hair in the process--definitely not as revenge or anything.)