Quick note that this is in the same timeline as "Still Here" and "Day Dream." Well, those were kind of generic so if you like to think of this as a stand alone series, that works too!
Two unusual things were playing out around Sans. One of them was splayed out on the couch slipping fruits into his mouth while humming a digitized rendition of the Waltz of the Flowers against a musical on TV. It went well together, surprisingly.
"Sans! Sans!" Papyrus gripped his younger brother's shoulders from behind. "Alphys called and said she needed a place to toss her friend in. Oh my god! I love being Alphys garbage can!"
"Oh? The doctor's robot friend?" Sans asked, knowing the answer already but also knowing that his big brother would have loved to answer.
"Not just Dr. Alphys' friend!" Papyrus shouted at the top of his non-existent lungs, proving Sans correct. Getting down on one knee in front of the couch, Papyrus pointed finger guns at their guest and introduced him. "It's Mettaton! Star of the Underground, Surface, and every nook in between! Singer, actor, miracle-bot, and best of all—" Papyrus sucked in a gulp of air. "—he can make PUZZLES!"
"That sounds swell." Sans said, making his brother wheeze and sputter in shock.
"Swell? You can't use mediocre feelings to describe him!" Papyrus yelled. "He's stunning, perfect, awe-inspiring, beautiful, gorgeous, fabulous-"
"Hey, Sans," Napstablook appeared beside the skeleton as the elder one continued his list, emphasizing his description with poses. "Can Shyren and I use your bathroom? The bathtub looks like a good place to lay down and feel like sludge."
"Aww, you never need to ask to do that."
"Thanks, Sans." Napstablook nodded, disappearing from sight again.
"And finally…" Papyrus announced, tapering off. "He's…he's…Uh there was one more word in the script!"
Leaning his ankle on Papyrus' shoulder, Mettaton did a standing split, placing his hands proudly on his hips.
"There is no 'final' word for me in any sense but you can put 'daring' on the list, darling," Mettaton told his footrest and the skeleton swooned. In a show of great flexibility, the robot leaned forward and whispered to the place where Papyrus' ear would have been if he had skin. "If you like, I can give you private lessons to ah lengthen that pretty vocabulary of yours."
If it wasn't for the fact that Papyrus lacked internal organs, he would have gasped in all of the air in the room.
"Wowie!" He pressed his hands against his cheeks in glee. "A private lesson with Mettaton? Even when I'm always struck dumb at just how amazing you are?"
"Hey, buddy," Sans uppercut Mettaton's crotch heart (well technically it was a little higher than that but close enough) and kept his fist pressed against it. "What were you doing here in the first place?"
"Kkk-Kkk-kKkk—"The static emitting from Mettaton's mouth sounded pained. "—W-why, t-taking shelter from m-my ad-doring fans!" He said, shivering but refusing to break his poise.
"Then why not have a seat? Being chased around must'a tired ya out—"
"I'm a machine."
In reply, Sans gave that heart button another hard bop.
"Being chased around must'a tired ya out. Why not sit down?" Sans offered, taking his hand off Mettaton's heart button, letting the robot let a breath out in relief. Sans punched him again before he could finish. "And stay sitting down."
"How ahh…hospitable!" Mettaton managed and Papyrus had been too busy being on the cusp of fainting in glee to notice his idol's discomfort. "Refusing such generosity would be utterly rude! I'll be taking my seat again." He finished, taking his leg off of Sans' brother.
Satisfied, Sans started on his way back to his room but Mettaton had decided to let his leg fall in front of him, blocking his way.
"But first—" Mettaton's smile glittered with glitz, one foot on the ground and the other resting on the sofa, cutting off Sans' escape. "I know my number one fan but who's the cute blue button living with him? How about we start off with a name? What's yours?"
"No." Sans answered plainly.
"Number one? I'm number one?" Papyrus exclaimed happily, the stars in his eye sockets were visible. "Oh and Sans?" His excitement deflated somewhat. "He's my lazy, gross brother who will always find a way to continue being lazy and gross…well not so lazy. He's still really gross though!"
"That's me." Sans agreed, winking his eye socket reflexively. That may have been a mistake because Mettaton winked right back.
"Oh, resourceful are you?" Mettaton grinned. "Lovely!"
That flashy floofy haired robot, despite doing the splits again, was leaning over Sans and the skeleton was trying to retract his skull into his hoodie.
Weirdly, Sans wasn't trying to escape into his own ribcage because being looked at that way for the first time was terrible. It was just…that. Weird. It made his head hot.
"Hey, Papryus? How long is he going to stay here?" Sans asked, words slightly muffled by his hoodie's collar covering his mouth.
"The twenty-eight hour death concert's been the talk of the news all day now! I wouldn't be surprised if the attention didn't die down in weeks!" Papyrus pressed his fingers together, hopeful. "Maybe even a month? Two months? More months? Mettaton living under our roof for a year? I'm going to die young and love it!" Papyrus collapsed into a heap just at the thought.
"…Eh, I don't mind living in the fridge for a year." Sans shrugged, heading in the opposite direction for the kitchen. Mettaton cartwheeled fast enough to intercept him and again, blocked his way while doing another perfectly posed split.
"But I'm much cooler than a fridge!" Mettaton boasted to empty space. Startled, the robot looked over his shoulder and to find Sans walking away. "How did you—Well then, it'll just be me and my precious fan! Maybe even some special one on one linguistics!"
"Oh! Oh my god!" Papyrus squeaked, bouncing up and down in joy. "Linguistics! I've never seen that kind of pasta before!"
"Really? Guess I'll stick around to see what's new." Sans plopped back on the couch and Papyrus groaned in frustration. Flipping the tv to the sci-fi channel, Sans pretended not to notice Mettaton sitting right next to him, crossing and uncrossing his legs several times.
"So, you like marvels of science, do you?" Mettaton stretched his arms back, showing off his sleek metal plating. "How would you like to see some more, free of charge?"
"Eh, most stuff like that probably but I can see some on TV too," Sans shrugged again, trailing off into silence for the next few seconds. "Wait…did you just…"
"Did he what? Do something fabulous?" Papyrus asked, climbing over the couch's arm to crawl onto the seat next to Mettaton. The robot was looking smug.
Despite himself, Sans felt his ever present smile widen just a little bit more and Papyrus noticed immediately.
"Free of charge…free of charge…Mettaton, you're a robot AND NO! GOD NO," Papyrus shouted in despair. "GOD PLEASE NO! NO!"
"It was obvious the moment I saw you, darling." Mettaton started leaning over Sans again.
"If it's going to be like that, a year wouldn't be so bad," Sans said, shoving Mettaton back with one hand. "But the personal space bubble does not get to be squeezed until month three."
"Ahaha..." Mettaton laughed, instinctively shielding his heart. "Fair enough."
"NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO-" Papyrus hollered and for now, the second unusual thing happening around Sans mentioned up in the beginning of this stretched out moment was lost in his brother's screams.
Among the many pairings in Undertale, this one jumped out at me more I guess? A really really obscure one but that may be because I love height differences (and maybe some 'Bop It!' jokes here and there). To be completely honest with you guys, I'm also playing around with Mettaton's character to get a better grip on him.
Besides that, I had a request come in for some angst regarding Sans and I haven't forgotten it! On that note, I'm planning a series with Undyne too! Uh wow, that's a lot to say with the basically zero free time I have but what can you do?
And hey, thanks for reading!