Geno is a Sans from another timeline who injected himself with Determination in Alphys's True Lab to hijack the resets, and now can retain memory for the other Sanses to use, though he himself is stuck in the void with Frisk. He is a main supporting role on Aftertale.
Geno Sans has Papyrus' scarf around his neck; he is very protective of it as if it is Papyrus him self.
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.
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 slipper 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 supposed 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.
- He has stronger versions of the original Sans's attacks.
- 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. He only dies if he leaves the loading screen (of his own AU).
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.
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.
After receiving his happy ending by the end of Aftertale, Geno receives glasses from Alphys. Eventully, through experimentation with his own determination, Geno sends himself to the anti-void. Through this, he slowly becomes an error, like all other beings who stay in the anti-void for too long. Once someone fully converts into an error, theey lose a large part of who they are, but their personality as an error is determined by the nature of their final thoughts before they became an error. In Geno's case, those thoughts were of one of regret, remorse, and bitterness for having done this to himself after receiving the happy ending he worked so hard to achievee, only to receive it by trusting in a more naive version of himself. Those thoughts caused him to hate quite literally everyone other than himself, making him miserable and lonely once his trasnformation was complete. In the end, he was his own undoing, although there are always other versions of him who got to live a happy life, and those that never got that chance. Likewise, there are versions of him who experienced something different entirely, such as Fatal_Error.