A little angst? S/O is very mad at skeleton for some reason, and eventually slaps him as a way to make him shut up. For the main boys.
Undertale Sans - Congratulations, you just lost his trust forever :D Sans flinched hard when you raised your hand and then just stared at you in complete disbelief and shock when you still went and hit him. This is one of Sans dealbreakers. He hates free violence, and more than that, he used to be a mistreated child. He is triggered, and he will get scared of you every time you make a wide gesture from now on. You messed it up so bad that you actually won't see him for an entire week, as he needs alone time to think. You probably won't like what he will tell you when he comes back. There's a high chance your relationship is over.
Undertale Papyrus - He looks at you with wide, scared eyes and pretty much literally throws himself on the floor, all his bones rattling in shock at what you just did. Great job on reminding him he used to be so scared of this in the past :D Papyrus is greatly disturbed, but he has a kind heart, so he will forgive you once. Because that has to be a mistake, right? But he is so disturbed that he will talk to his big brother about it, and oh boy, you won't like having Sans shortcuts in the middle of your living room with pitch-black, very angry eye sockets and telling you, eyes in eyes, that if you ever do this again, there will be consequences. Papyrus is nice, but Sans owes you nothing. He's nothing but hostile with you from now on.
Underswap Sans - He jumps a bit, then stares at you in confusion. He immediately looks down, feeling guilty. He knows he can be a little intense sometimes, and he actually quietly apologizes, not daring to meet your eyes again. Blue doesn't take offense at this; he sort of deserves it, right? Yet, he will stay up all night thinking about this, and trying to convince himself that it's ok, it was only one time, no big deal. Right?
Underswap Papyrus - He just gasps in shock and flinches when you take a step to apologize. Honey just shakes his head, refusing to talk and shortcuts out. He hates violence with all he has, and it hurts way more than physically. He will forgive you eventually, because he's a really nice guy and he loves you, but never do that again. Because he won't be as forgiving after this. You're actually lucky he didn't tell Blue, because Blue would have forced fate and thrown you out of the house himself. Blue is suspicious, though, and noticed his brother is acting weird. Watch your steps.
Underfell Sans - In his world, it's an eye for an eye. Yeah, good job, you slapped him! He's biting your hand until it bleeds, growling angrily, and this will need a hospital visit to heal. Red is not the type to get hit and act like nothing happened, though. He has huge resentment, and he doesn't want to see you for a few days while he's cooling off. He's just so angry right now. Only then might he accept an apology, but he will be wary around you, and he's clearly not ready to trust you again. He might again if nothing else happens, but you're on thin ice, and it might end with him breaking up with you.
Underfell Papyrus - He takes it bravely without even a flinch, but his eyes are screaming disappointment. He thought you were different than the others. Turns out you're all the same, humans and monsters alike. He's disgusted. He tells you very truthfully that he knows he can be a loudmouth, but that he's listening and that talking would have been enough. There's no turning back now, however. He might forgive you this time, but he will be wary around you, and he will refuse any affectionate gesture coming from you for a long time. He doesn't trust you anymore. It will most likely end with a break-up.
Horrortale Sans - He stares in shocked silence, looking at you like a wild trapped animal. Like Sans, you just broke his trust forever. Oak had been hurt too many times to forgive, and now he's just scared of you. If you're lucky, he might forget, but Willow will notice he is agitated and will look in his notebook to understand, finding out what you did eventually. Well, that's easy: get out of his farm :D He doesn't want you here anymore. Hitting him and then acting like it didn't happen is the worst you could have done, and if Oak forgave you against his will, he won't, and he's here to make difficult choices for his brother. Leave. Never come back. If you do, he will sue you and ask for a way to keep you away from his brother. Willow is done being nice to the wrong people.
Horrortale Papyrus - Willow gives you a very hurt look, then simply looks away in disappointment. You almost made him believe something good could happen to him, and here you are. He smiles, trying all he can to not cry, and just tells you that he forgives you, but that he can't be your partner if this is how you react to slight frustration. He doesn't hate you, but he can't keep living like that. He can't let people hurt him again and just act like it's fine and it doesn't matter. He is not Papyrus anymore. He has boundaries, and if you can't accept them, then it's best you two part ways.
Swapfell Sans - Nox immediately shuts down, looking very small. He won't complain. He won't say anything, actually. He's just staring at his feet and apologizing again and again in a whisper to stop making you mad. It's working with Toriel. It will work with you, right? He flinches when you put a hand on his shoulder to make him snap out of it. He will quickly go back to how he is usually, acting like that never happened, but he has that bit of concern on his face every time you call him or walk toward him without saying anything. He won't complain. He never complains. But you can tell something broke that day between you two.
Swapfell Papyrus - Well, not cool. Ok, he pushed a little too much, but you don't have to hit him to tell him, jeez. Rus calls you out pretty much immediately and sets very clearly that he doesn't want it to happen again. He will forgive one time, because, well, he can tell he pissed you off, but that will be only one time. He doesn't need more violence in his life, thank you.
Fellswap Gold Sans - Well, he has strong instincts. You slap him, he slaps you. It sort of came instinctively. He's not too mad about it, as, well, you both played and you both lose, but it better not be something that happens regularly or he will have a lot less patience about it. And well, when Wine loses patience, the result is rarely a good one.
Fellswap Gold Papyrus - Coffee turns silent and stares at you, looking both shocked, very hurt and terrified. You try to apologize, but he just shortcuts away the second you take a step his way. He turns non verbal for long hours, crying in his closet because he feels so stupid. He lowered his guard, and that's what happen? Hell with that. Well. Hell with you as well because Wine will definitely notice Coffee is upset, and, if Coffee refuses to tell what's wrong, he will search himself and he will very much not appreciate seeing you slap his brother on one of the security cameras. Good luck with that.
