Before the human arrived in the Underground, Sans was more of a lazy and friendly skeleton who would often make jokes. He would go to Grillby's for a good amount of time; he was very caring about his brother, Papyrus. However, after the human reset and did numerous Genocide Routes, one day due to unknown circumstances Sans suddenly remembered all the Genocide Routes. This changed Sans, and made him kill other monsters for LV; even going as far as killing his own brother.