Geno!Sans is a 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 . 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.
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 can't die. He only dies if he leaves the loading screen.
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.
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