Sans is the brother of Papyrus and a major character in Undertale. He first appears in Snowdin Forest after the protagonist exits the Ruins. He serves as a supporting character in the Neutral and True Pacifist routes, and as the final boss and heroic antagonist in the Genocide Route.
Vote which fictional character would naturally fit in-universe with a big-named franchise! No time-travel, no interdimensional crap, no "Pooh's Adventures", they would just exist in-universe.
