I got a really long ask so I'm going to split it into multiple posts :D
What are the underjumble cast's pronouns?
All pronouns are not required to be used by characters unless stated otherwise, have fun tweaking or adding on!
Sans hasn’t really questioned his gender. He doesn’t feel happy OR sad when he hears he/him. It feels right yeah, but he feels neutral about them. I don’t think he would mind with any other pronouns, it would be a bit huh? but at the end of the day he’s not gonna feel bad about them unless it/its. He uses He/him.
Papyrus is too young to have even considered the idea that he’s not male. He/him.
Asriel is, you guessed it, he/him. Sorry y'all. I’m infamous for making all my characters queer but not right now.
Chara uses they/it. They’re referred to by them a lot, but some pepper it in. They don’t mind if you call them exclusively by one or the other. Chara is canonically non-binary (not as in the gender itself, but as in outside the binary) in underjumble, and you may not refer to them using he or she. Any others are fine.
Napstablook is a little silly they/them! They will not correct you if you don’t use the right pronouns due to anxiety, though, so I won’t take it as misgendering if any character uses pronouns other than that, since it’s usually Mettaton standing up for them. Napstablook is a bit ambiguous because although they use they/them they are a demiboy, so I will accept using he. Just, don’t make them into a he/him only, okay? I want to respect their original pronouns.
Mettaton is a he/him. Specifically, a trans he/him! :D I would prefer if you kept part of his masculine pronouns, but he’s not outright confirmed to be trans in undertale, so I won’t keep that rule with an iron fist.
Maddie, like in canon, is a trans she/her catgirl. You cannot use he for her.
Toriel is a she/her
Asgore is he/him. Sorry but he’s the embodiment of a 40-50 year old divorced depressed cishet dad. Love him though!
The sapphics are she/her :)
Gaster is a he/him. He gives me the vibes that he wouldn’t be purposely transphobic, but he’d make mistakes that make you cringe, you know. His heart is in the right place.
Frisk is they/them. He or she will not be tolerated.