Undyne paused behind the doorway. Frisk and Sans were behind it, she had seen them both enter a little bit ago. She hesitated to go in, not wanting to see the couple in a compromising position, but Papyrus wanted her to fetch them to try out his latest batch of spaghetti. If she didn't go in to get them, he would just drag them out, uncaring of their states and Undyne would get an eyeful regardless. That hesitation let her overhear a conversation that would shatter her world.
"Can we go out on another run please, Sans" Frisk asked. She was a darling thing, and everyone loved her. There were more than a few monsters jealous when she started dating Sans, who everyone knew was an annoying slacker instead of them. Undyne was happy to count herself and Papyrus out of that horde, as Papyrus had already dumped them long before she started a relationship with anyone else from the underground and she wasn't interested in a human, not with all of the anti-human training that she still loved.
"Frisk, already? We just did one not that long ago" he complained. This was interesting. Sans didn't go out and do anything, being a complete lazybones, aside from fulfilling their minimum requirement at several jobs. Undyne was sure that someone would have noticed if Sans had done something so out of character.
"But I want to do another one" Frisk whined. If there was one thing that Undyne still hated about that human it was how demanding they were. Intrigued, Undyne stayed quiet and listened carefully. Was she going to be the first to know how the cute human managed to order Sans about successfully? Not even Toriel, Queen of the Underground and someone who had known Sans for years before her failed attempt to make Sans her consort, had ben able to make Sans do anything. It was a huge surprise to the Underground at large when he had started taking orders from his girlfriend, especially with the way his falling out with Toriel had gone. That had been one messy affair. Sans had become Toriel's consort for a few years before he had in a very public blowup refused the position on the grounds that she was to pushy. Sans hadn't pulled any of his punches in that argument, unafraid to say that he was tired of her hiding her feelings for the King and his love for her with their fling. He even called it a fling to her face, unafraid of who would overhear. There had been rumors going around that Sans was afraid that the King would finally take some action against his wife's abandonment of him, but when he started dating Frisk a few months after that, no one could accuse him of cowardice. He had fought hard to keep his claim over her, including an escalated fight with both the King and Queen at once. He was still getting side glances for having been able to win against both of them at the same time, and without Frisk intervening.
"You still know that I hate doing them, kid" Sans retorted after taking his time. It seemed that his humor didn't get him out of things when no one was looking after all, not in the way that he had fended off Toriel's requests at any rate.
"Do you want me to carry through on my threats" Frisk said abruptly, in a suddenly stone-cold voice, devoid of the mercy and forgiveness that she was so well loved for. There went that idea that Sans dated her because she didn't actually hold him to his promises. That was definitely holding her ground, in that tone it nearly was threatening.
"No" Sans backtracked quickly, sounding afraid. What was it that could make Sans, judge and strongest monster in a fight afraid? "Please don't, I wasn't trying to back out" It almost gave Undyne whiplash just listening to how quick the mood went. One second, he was resigned, and the next he was groveling? That didn't make sense.
"Because I could still do it you know" Frisk continued, pressing regardless of Sans fear. That seemed out of character to. Still, the next thing that Frisk said shocked Undyne. "It wouldn't be that hard to launch a surprise attack on Papyrus, I could easily corner him, he wouldn't even fight it until it was to late. I could easily revert if you back out Sans."
"I know" Sans said, sounding pained. "You still could just start a massacre here, you dirty duster"
It was the first time that Undyne had heard him use that tone, and the address was just out of place for innocent Frisk. The chuckle that Frisk gave wasn't the denial that Undyne had expected, however.
"Cheer up, Sans, sweetheart. I like dating you. I like being part of the family, and trying out Papyrus's near poisonous pasta, and all those other things that this gives me. As long as you keep going with me to fill darker urges on other timelines, this one will stay safe" Frisk sounded unrepentant. Looking through the crack in the door, Undyne saw a dust covered knife being fiddled with by Frisk, who's hands were gloved now, the gloves just as guilty as the knife. That was the work of several attacks. Those threats were genuine.
"I wish that you would just threaten me when you wanted something like that" Sans grumbled. "I wish that you would stop with the threats and the runs altogether"
"But threatening you brother is so much more effective" Frisk purred, "and I like seeing us fight together. Your so strong, Sans, and I like to see that strength at my side, just as guilty as I"
"Should never have gone along with this deal" Sans grumbled, changing his outfit so that his blue jacket wouldn't get dusty. Frisk passed him a dust tinted one in a lighter blue, and Sans truly looked just as guilty as her.
"But your just so protective of Papyrus" Frisk teased, her point long won.
"Not going to let you become a dirty brother killer, Frisk" Sans said, apparently desensitized, snapping back to his normal punny self. "Love both of you too much for that, love every bone-iful part of your con-tibia-ed lives"
Frisk just chuckled, looking sinister as she dragged hold of Sans and went somewhere off with him that Undyne couldn't see. She just tried to forget that horrible conversation, even though she never left Papyrus completely by himself anymore and was forever suspicious of that couple by now. How could she ever trust them again when they did that behind closed doors?