The story takes place in an alternate timeline, where only 2 humans ever fell in the underground, Sans and Papyrus are just friends and a lot of monsters are deceased. After the death of the second human (Frisk), a new human named Cherry fall into the underground. Thinking that Cherry might be dangerous, Sans attempts to attack her, but Papyrus protects her, which annoys Sans. A while after that, another human named Jewel falls. Sans is more respectful to Jewel, but Papyrus, feeling that Jewel is potentially dangerous, and attempts to kill her, which Sans prevents. After that, Sans and Papyrus lose their friendship and each become father figures to the humans they protected.
Insanity Sans looks like regular Sans, but with some differences. Sans' jacket is neon green with a black hood and zipper. His T-shirt, and slippers are brown, with his slippers worn with black socks and his shirt with a tear in it. His shorts are a different shade of brown and have gold stripes. He is always depicted smoking a cigarette and having his eyes ignited. His right eye represents his good side, being light yellow with a green outline, while his left eye represents his evil side, being purple with a red outline. His left eye is bigger, meaning he is more evil, but overall he is neutral.
Sans is often serious, not letting people force him around causing him to be incredibly feared by a lot of monsters. He usually isn't good with communication or socializing, only being friendly to people he can trust. Even if he is only social with people he can trust, he can become enemies with some people easy, including friends. The only person truly cares for is Jewel, who he will protect under every situation.