Geno!Sans is a Sans from another timeline who has gained enough determination (by injecting himself with it in Alphys's True Lab) to hijack the resets, and now can retain memory for the other Sans' to use, though he himself is stuck in the void with Frisk. He is a main supporting role on Aftertale, which is now complete. It also happens after the Genocide Route after you kill sans.
Aftertale Sans is a Sans who was near-death in a Genocide timeline. Calling himself "Geno", he lives in the save screen with the original Frisk. Geno wants to completely destroy the timelines, but as he only has a fraction of a soul, he tries to get other Sans' to do it. If they don't agree, then he will erase their minds and starts over again. Recently, he stopped doing this so that the current Sans will remember all the resets and Genocide routes in the previous timelines in hopes to get him on his side.
Geno "survived" the Genocide timeline by injecting himself with determination and saving as he died. Geno lives in the Save Screen with Frisk and, if he leaves it, time would continue for him and he would turn to dust. However, he can travel to the anti-voids without risking death.
Description of Geno EditGeno Sans has Papyrus' scarf around his neck; he is very protective of it as if it is Papyrus itself.
Geno Sans has a slash on his white jacket from his fight against Chara/Frisk. Geno Sans' right eye is covered by a glitch, probably to hide the part of his face melting from the determination he injected himself with.
Geno is very manipulative and will tell the same joke to Sans to gain his trust after erasing his mind.
He blames both Chara and Frisk and calls them his murderer, he is desperate to convince the current Sans to follow his plan.
He is also very sensitive about Papyrus and will not let anyone touch his scarf. When he reunites with Papyrus he is furious at Sans for bringing him into the save screen and making him fight in front of him. However, he still has a sense of humor, which is good news for us all (I guess?).
Like the original Sans, Geno Sans has an unzipped hoodie jacket with a shirt underneath, along with short pants and fluffy slippers. However, he has some differences to every other Sans. Geno Sans's jacket and slippers are completely white, and since he's from a Genocide timeline, he has a huge bloody gash that Sans gets after he's defeated in the Genocide run. His slippers are also bloody and he wears his timeline's Papyrus scarf, which he is very protective of. Also, because he wasn't suppose to be able to enter the loading screen, he is seen with white glitchy particles around him, one of which is covering his right eye socket. Due to having injected determination into himself, it is believed that Geno Sans placed those particles there on purpose to hide the part of his skull that's melting off.
During a continuous cycle of Genocide routes, Sans watches a recording of Papyrus being murdered by Chara, which then made Sans lose himself and inject himself with determination found in Alphys' True Lab. With determination, he fought with Chara, but finally lost after 536 tries. He then teleports to the loading screen and is saved as so, but cannot escape from there and is stuck there with Frisk (whose body is being controlled by Chara). He takes care of Frisk in the loading screen and allowed Frisk to play with his Gaster Blasters. He got new powers, but the scar of the fatal blow still remains there. He calls himself Geno, as a reminder of how he failed to save everybody in his timeline. He tries to get the current Sans to help him with his plan to erase their timeline so Chara can't hurt and kill Papyrus and his friends anymore. When Sans refuses he forces Sans to see the worst timelines and waits for him to agree to his plans. Sans realizes that being alone makes 'Sans' worst he tricks Geno into bringing Papyrus, him and Chara into the saving screen. Geno attacks but Papyrus manages to convince him to try Sans's plan of giving the power to reset back to Frisk. Chara confronts Sans about the other missing children before Frisk convinces them to rest in peace. Frisk resets and later gets the idea to give the butterscotch pie to Geno, allowing him to live on the surface with Papyrus, Sans and the rest of the cast. However both Gaster and Chara warn that life on the surface won't be perfect and that Frisk will reset. Geno says that if this plan fails he will continue to try and erase their timeline but he gains hope and thinks that life on the surface with his family is the best possible ending.
Geno Sans normally uses Gaster Blasters but consumes a lot less energy in that attack.
(His attacks are similar to the UT Sans but he is way more powerful.)
He can control time for a few seconds.
He can RESET the timeline and visit other alternate universes.
Geno!Sans can also become invincible for a short amount of time by using glitches but it takes a lot of energy.
He can't die unless injured, he was given a butterscotch pie from life Toriel (ReaperTale) in which altered his health to a point where he would be able to roam around with out his HP dropping and himself dusting.
His mission is to save his timeline by stopping Chara and thus killing them, ensuring everybody will no longer get the suffering they don't deserve. However, he cannot do it himself because, if he leaves, he dies and Chara will still be in control. So, he tries to convince Sans to bring Chara to the save screen to get them out of the way. He will then attack them until they fall into a coma and then teleport out of the save screen to end the timeline because he will then be the one with the most determination, and therefore have control over the timeline.