Geno!Sans is a Sans from another timeline who injected himself with determination from Alphys' True Lab in an attempt to hijack the resets. He is now able to retain memories of prior timelines 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.
Geno is a Sans who was near-death in a Genocide timeline. 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, sending him to the save screen, held in a state between life and death. 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.
Geno Sans has Papyrus' scarf around his neck; he is very protective of it as if it is Papyrus itself.
Geno Sans has a bloody slash on his white shirt from his fight against Chara/Frisk. Geno Sans' right eye is covered by a glitch he creates to hide the right half of his head, which has melted because of his determination.
Geno is very manipulative and will continue to manipulate Sans to gain his trust after erasing his mind.
He is also very sensitive about Papyrus and will not let anyone touch his scarf. When Sans brings Papyrus with him on his visit to the SAVE screen, Geno is furious. However, he still has a sense of humor. He is also very emotional.
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 white 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, Geno Sans placed those particles there on purpose to hide the part of his skull that's melting off. (however he has shown Fell sans his eye in Christmas Party Au - not shown to people) he does this to ensure that "Sans" would not be frightened. He also has blood dripping from his mouth.
- He has stronger versions of the original Sans's attacks.
- He can't die. He only dies, if he leaves the loading screen (of his own AU).
- He can bring other people into the SAVE screen
- He can erase and manipulate people's memories
Geno's relationship with Sans is complex. As they are technically parts of the same person, Geno does care about Sans's well being. However, he is not above manipulating Sans to get his way. Sans is an integral part of Geno's plan to erase the timeline, and as such, Geno is willing to do whatever it takes to win him over.
His relationship with Frisk is a little bit complicated. When Frisk first entered the SAVE screen, Geno intended to kill them, or to come as close as he could within the SAVE screen. However, he decided that they could be a useful tool to recruit Sans to his side, and spared them. After that, his relationship with Frisk seemed to be relatively good, with him allowing Frisk to play with his Gaster Blasters.
It is unclear exactly how much of their relationship is faked by Geno to gain Sans' trust, but it would appear Geno saw his mercy as a second chance for Frisk, and an opportunity to repay them.
Geno loves Papyrus, almost to the point of mania. He wears Papyrus' scarf around his neck at all times, and refers to it as Papyrus in some instances. He cares deeply for Papyrus, and his goal to erase the timelines is fueled by the belief that that is the only way to stop Papyrus from suffering.
Geno hates Chara, holding them and Frisk equally responsible for the Genocide runs and resets. He attempts to kill them when Sans brings them into the SAVE screen, and is furious when Sans protects them.
Error!Sans (Future Geno!Sans)
Error!Sans is a version of Geno that was thrown into the Anti-void as a result of messing with his excess Determination. Error has no memory of his past as Geno, and as a result views him as nothing more than another anomaly. In the comic Fatal_Error, Error attempts to kill Geno, but because you cannot die in the SAVE screen, the code glitches transforming him into Fatal_Error.
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, seeing it as the only way to end suffering, but as he only has a fraction of a soul, he tries to get other Sans' help to do it. If they don't agree, then he will erase their minds and starts over again. During the events of Aftertale, 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 Frisk, and gaining control of the timeline, ensuring everybody will no longer suffer. 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.
Epilogue (Prologue to AskError)
After receiving his happy ending by the end of Aftertale, Geno receives glasses from Alphys. Eventually, 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, they 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 achieve, 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 transformation was complete and pushing him to hunt down every single AU to destroy them. 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.
- He also appeared in the "Christmas Party AU" comic. But for some reason, he doesn't enjoy the party.