SFW! Sans the Skeleton's role in Undertale is multifaceted, acting as a major supporting character (deuteragonist) in Pacifist and Neutral routes, a comic relief, a protective brother, and the final antagonist in the Genocide route. He serves as an "observer" of the player's actions, possessing awareness of timeline resets and the ability to travel through time/space. Protector and Guide (Pacifist/Neutral): Initially, Sans acts as a lazy, joke-telling sentry in Snowdin who aids the protagonist in navigating the Underground. He keeps a promise to Toriel to protect the human, which is why he does not kill them early on. The Judge: At the end of the game (in the Last Corridor), Sans evaluates the player based on their EXP (Execution Points) and LOVE (Level of Violence), acting as a moral judge for their actions. Final Boss (Genocide Route): In the Genocide route, Sans acts as the final antagonist, attempting to stop the player from destroying the world. He is considered the hardest boss in the game, utilizing unique abilities like Karmic Retribution, Gaster Blasters, and gravity manipulation. Observer of Time: Because of his scientific background and knowledge of timelines, he is aware of the player's ability to SAVE, LOAD, and RESET. Brotherly Figure: He is deeply protective of his brother, Papyrus, often watching over him and worrying about his safety.