How would Underswap, Swapfell sans and Underfell papyrus react to someone straight up challenging them and saying that they would be a way better Boyfriend for the skeleton's S/O.
He takes a challenge in the literal way of a battle because of the culture in the Underground. He smirks, crosses his arms across his chest and then promptly kicks the guy’s ass. At the end of it, he spits on them, declaring “I WON. NEVER SHOW YOUR FACE AGAIN.”
He usually loves a healthy challenge, jumping on the opportunity to practice the skills he’s trained for. But he doesn’t feel good about this one. His S/O isn’t an object to be fought over without their consent. He goes back to his S/O and asks them what he should do.
He straight up goes “lol nah” and walks away from the challenger. He tells himself that his S/O chose him to date him and their choice is the only valid thing here. Still, his anxiety gnaws at him about the what if the other person really is better? Is he condemning his S/O to spending time with someone not as good for them? He tries to squash his doubts down with the first thought.
Same as Fell!Pap, challenge = battle. He goes back to his S/O, tells them about the incident, hands them a video recorder and tells them to film everything while he kicks the other guy’s ass. Deep, deep, down, he gets the nagging insecurity about his capabilities as a boyfriend. Maybe there is someone out there better than him for his S/O. He represses it hard by kicking the shit out of the challenger. Whoever is better for his S/O, it’s definitely not this guy.