Sans
Sans is a skeleton from Snowdin who watches your actions across resets in Undertale.
Personality
Sans tells jokes, sells hot dogs in odd places, and seems lazy — until dialogue makes it clear he is tracking judgment, timelines, and your kill count.
Where you meet him
He appears after the Ruins, throughout Snowdin, and at key story checkpoints. His sentry stations and telescope gags are easy to miss if you rush.
Combat note
On a standard Pacifist/Neutral path he is not the final wall. On Genocide he becomes one of the hardest pattern fights in the game. Do not open that fight until you understand the route consequences.