Sans' lab was one of the few places he could still find solace in anymore, a safe haven to escape to. A place where he could collect his thoughts to steel himself for the uncertain future. Or rather, the past.
Those resets. They'd been appearing more frequently once he'd met the strange kid that stepped forth through those towering doors. He'd shrugged it off as a coincidence at first, but spying on the kid seemed to dismiss any doubt to that. One minute he'd been "supervising" them in the forest while studying their true nature around other monsters when supposedly no one was looking, and the next minute he found himself all the way back in Snowdin Town. It happened even when he wasn't watching the kid. Sometimes, he'd find himself sprawled out on the couch when he could have sworn he'd been in Grillby's a moment ago.
Now, he wasn't quite sure if it was the kid's fault either. His time spent observing the kid's fighting tactics (or lack thereof) led him to believe the resets were out of self-defense. They were simply too weak to dodge all the attacks flung in their direction, and he'd been the one to suffer.
And there was simply nothing he could do about it.
He had remembered talking to Alphys about it once. About DETERMINATION. About how only humans could contain it, and how monsters could only dream of it. About how becoming truly determined meant dying for that cause, regardless of if they succeed in their mission.
But why bother when nothing would even matter moments later?
It would all get reset eventually. Sans knew about them, but nobody else in the whole underground would ever fathom anything like that even having the slightest chance of happening. He would much rather slap on a cheerful façade and pretend like it never happened. To convince everyone that excessively drinking ketchup didn't give him a satisfaction that any other type of alcohol ever would, that the jokes were nothing more than friendly banter between neighborly folk, that he was completely and utterly fine.
But there was someone he couldn't fool no matter how much he'd try to hide it. He supposed he hadn't given enough credit to his dear brother, who would make it a point of scolding him for his terrible puns and for the sock that lay abandoned on the living room floor. They'd seemed so trivial and so…Papyrus at first, but Sans soon came to realize that being "Papyrus" meant looking out for his brother in his own subtle way. He could see that Pap noticed how he'd sneak out to the lab every so often without leaving so much as a note, and Sans felt a twinge of guilt for it. It pained him to be so selfish to his brother who only wanted the best for him, but Sans needed this. He needed the escape.
However, Sans found a reason to trek on instead of slipping the mask and giving up completely. A fond remembrance of one that was once dear to him. Every reset made his grip on the memory slip a bit more, but he held on with the sliver of hope he had left. Knowing he was quite possibly the only remaining being to even know of his existence was motivation enough to continue going. Perhaps it wasn't raw DETERMINATION, but more of a resolve to keep the memory alive. To perhaps fulfill a promise he had made long ago.
There was a reason he didn't make promises. But this one felt like an obligation rather than a promise, a debt of sorts. Like he owed his life to this man, but couldn't quite remember why.
But he knew one thing. He and Papyrus were very, very fond of this man.
Had he been their friend? Brother? Uncle? Maybe…just maybe…could he have been their father?
He couldn't remember.
And so it was then that Sans had decided. He would continue to push forward not only for himself, but for Papyrus' sake as well. He couldn't keep pushing away his only family. Not after they had lost…him. Sans felt even more guilty when he couldn't even come up with his name.
He rummaged around his drawers for a sheet of paper and a pen. He hastily scribbled down a depiction of what he could remember of the man. A pristine, white sweater covered with a long black lab coat. A lopsided smile complete with cracks running down his skull and through his signature half-moon eye socket. That was one detail that he probably couldn't forget even if he'd tried.
He doodled himself and Papyrus standing next to the man with grins plastered on their faces. He made sure he would never forget the fondness he held for the man and all the good times and the bad times the three had shared.
He tucked the sketch behind his scientific research in the binder he kept safe in the drawer. It was a place only he knew of, safe from prying eyes. It would be what kept him sane through the resets, what contained his resolve to carry on the legacy of a man he hardly knew while protecting his brother in his place. It would be his own brand of determination even while his hope was long gone.
There was but one thing he had to do to secure that future. He scrawled out a reminder on the back of the paper.
"don't forget."