The character personalities are changed slightly or "Sans-ified" but other than that the story remains the same. This AU takes place after the events of the Undertale genocide timeline where Sans survives the battle with Chara and uses the last of his magical energies to clone himself and send the clones to merge with an alternate timeline in the hopes of preventing the catastrophe. The Sans clones merge with the characters from the alternate timeline and "Sans-ify" them but have no memory of the previous genocide timeline. All the Sans (Sanses or Sansi) have an appearance identical to the original Sans but with slight variations in color and clothing.
- Frisk! Sans (Frans) is the protagonist of Undersans. He is the last of the eight huskeletons (human skeleton/hybrid) a fall no underground. FRISK! Without wearing a striped hoodie blue and pink with blue pants and brown shoes. He is usually calm and curious but loves to make puns and flirt.
- Flowey! Sans (Sansey) is the main antagonist of Undersans. Wears a green hoodie with a few yellow petals around the hood with a yellow shirt and shorts and green shoes. Flowey! Sans is very bad and is always trying to kill Frisk! SANS and make them "have a bad time", but also loves puns about death and dying.
- Toriel!Sans (Sansiel) is one of the first characters encountered in Undersans. He saves Frisk!Sans from Flowey!Sans and acts as a protective parent towards him. Toriel!Sans is a skeleton covered in fur with horns and ears. He wears a purple hoodie with the delta rune on it and a white shirt with purple shorts and shoes. He loves baking, eating snails, making puns and being a mom.
- Sans!Sans (Ssaannss) is a doubled version of Sans and the brother of Papyrus!Sans. He is the only character who knows about the other timelines. He looks exactly like sans but twice as big and does nothing ever because he is so lazy. He likes being lazy, making lots of puns and making people have a bad time.
- Papyrus!Sans (Sanyrus) is the younger brother of Sans!Sans. He wants to capture a Huskeleton so he can join the Royal Guard. Sans!Papyrus wears a yellow hoodie with a red shirt, blue and yellow pants and red shoes. He likes spaghetti and making puns and will go insane if his brother is hurt.
- Undyne!Sans (Sansdyne) is a warrior for the royal guard. He wears the same clothes as normal Sans but they are gray and made of metal. He wields a blue spear of light in the shape of the gaster blasters and loves to fight and kill all the humans and make people have a bad time. Undyne!Sans dies without water. Also, he loves fish puns.
- Alphys!Sans (Salphs) is the royal scientist and the creator of Metaton!Sans. He is a yellow skeleton that wears a white lab and glasses because he is a scientist. Alphys!Sans loves science, watching anime like Mew mew Kissy Cutie 2, and making anime puns.
- Mettaton!Sans (Smettaton) is a smexy (XD) pop star of the Underground who was created by Alphys!Sans. He is a pink and black robot skeleton who loves to sing and is a gameshow host. Mettaton!Sans likes cooking, being smexy, and making puns for the viewers.
- Asgore!Sans (Sansgore) is the king of the underground. He is a big skeleton with ears and horns. He wears a purple hoodie, with shorts and shoes made of gold, and a golden crown. Asgore!Sans wants to take the soul of Frisk!Sans so he can free the monsters from the Underground.
- Asriel!Sans (Asansriel) is the son of Sansgore and Sansiel. He's a short skeleton with white fur, goat horns, and a goat skeleton as a body. Like Charans (foreshadowing), he wears a yellow-green striped sweater. In True Pacifist he becomes ASANSRIEL DREEMURR. What does he look like? Well, imagine Ultra Sans and ASRIEL DREEMURR. Then fuse them. That's what he is.