`RED`Aftertale Sans Bio

Sans was just like any other Sans, going through these resets like any other Sans, until he met a certain other version of himself by the name of Geno. Several times, Geno had tried to get Sans to 'help' him defeat the evil causing the resets, but the way to help, according to Geno, was to destroy the timeline with no exceptions, with the intention of never letting anyone suffer the resets again. When this was revealed prematurely during one attempt, Sans snapped and lashed out against Geno, telling him that his attempts were futile and to stop trying, not wanting anyone he cared about to be slaughtered by the likes of him. Although, it was a lot more aggressive than that. With his self-esteem and motivation in the negative values, Geno stopped after that, and Sans never saw him again as he seemed to disappear and not try to bring Sans back to the save screen. As it turned out, Geno ended up leaving the save screen entirely by being invited to the Omega Timeline, and was able to find happiness there. But that's a story for another time...

The resets and genocides continued, with Sans slowly losing his sanity from seeing his friends and family die over and over. They seemed endless, ranging into the hundreds in numbers. Until one day there were no resets. And the human hadn't shown up. Curious about the whereabouts of them, and in denial that things would finally be changing for the better, he went on a search for them. That's where he found a grey door in Waterfall. He opened it, and the human's body was found laying there. Dead. Soulless. Sans panicked, not knowing what to do, not remembering this ever happening. He went straight to Papyrus, ready to try protect him from any threat that might have come, even getting a bit controlling of him and obsessive for his safety. And his surroundings started glitching a few hours later. Glitching and glitching. And then he, along with Papyrus, were in the save screen once again.

 They both witnessed the one and only Geno, nearly dead with two slashes on his body instead of one and with a goopy, demented version of someone who he could have sworn he would never forget, but he couldn't remember why. Sans realized Geno was back now; this could mean he could have another chance at help to finally get the suffering to end. The brothers fought on Geno's side, and they managed to defeat him and lock him back into the void forever. With a heightened amount of Determination, Geno performed what became known as the Redact, which reset the timeline back to normal and fixed all that the goopy Voidster had managed to break, redacting everything back to how it was before the human even fell, and even managing to return the real Frisk, who had been sitting in the save screen helplessly, control of their soul and body. With the golden opportunity present, Sans begged Geno to be present again so he could help end the resets, so bent backwards that he didn't even care if it meant going back to Geno's original plan. But much to his dismay, Geno declined, wanting to be with his new family and having gotten over Papyrus's death. He sent the two back, ultimately leaving them with the additional salt to the wound of disowning Papyrus as his brother and claiming total allegiance to his new family. And Sans has never seen him again.

More stuff happened (explained in Chara's bio) and the last true pacifist run was completed, bringing all of the monsters up to the surface, including Chara. Sans, obviously, was less than pleased that Chara was accompanying and staying with them all, but Frisk's insistence to give them a chance kept him from obliterating them. And only Sans, Chara, and Frisk lived with the memory of what happened in the save screen.

The Redact didn't fix everything though. Sans was created from Geno once Geno had cheated death, prompting Sans to be created with 8/9 of Geno's soul once Geno was ejected. But since Geno still existed, the 1/9 that Geno had was never given back to this Sans. Even on the surface, Sans still exists with only 8/9 of a soul, and with no extra boost to sustain like a normal monster, unlike Geno who was able to utilize his Determination (and the fortune of not dying when he ran out of the save screen in the first place) to sustain a mostly normal life afterward. The lack of a full soul, plus how weak and frail his body was anyway, began to chip away at his health, causing complications that interfered with his day to day life that got worse as the years went by. While they were underground, his condition was being brought back to normal as each reset occurred, like how other monsters came back from the dead and went on as normal. The issue with being on the surface was that there were no resets. His health gradually kept deteriorating and there was nothing conceivable that could be done to bring him back to normal. Monsters were too weak to perform transplants, and human souls would make him godlike and risk overloading his fragile body with magic, leaving him with nothing but supplementary magic (which still didn't make up for what was lost) and the occasional magic booster for serious complications (similar to an epipen) for temporary solutions to his problems.