Sans Undertale

Sans is lazy, often sleeping on the job and taking breaks. His laziness is a combination of fatalism and apathy, as he is aware of timeline resets. Conversely, he suggests that this knowledge could be "a poor excuse for being lazy."[3]

Waterfall location Sans telescope Sans's fake telescope.

He enjoys being mischievous. Sans primarily acts this way towards those he is in good terms with. His antics range from an assortment of illogical, juvenile, and slightly inconvenient pranks. Examples include Sans "selling fried snow," using whoopee cushions, sharing an unscrewed ketchup bottle, and eye paint on a telescope. He is not above utilizing his teleportation or other magic for practical jokes either.[4] Sans is also fond of making the occasional low-brow puns. He enjoys making others laugh in general, as he also performs at the MTT Resort as a comedian.[5][6]

Despite Sans being apathetic towards his own life, he is considerate to those he cares for. For example, Sans is a regular at Grillby's, and the monsters and Royal Guard dogs treat him kindly. Sans has informed Big Mouth about "all kinds of incredible foods," yet Big Mouth comments that "he always orders the worst burger off the menu." Although Sans hates making promises, he respects Toriel's request to watch out for any humans.[7] Even if the protagonist killed people that were close to him, Sans refrains from broaching the topic until the Last Corridor. In a Neutral Route ending where Papyrus or Toriel is alive, Sans omits the protagonist's slaughter to either of them.

He is also observant; Sans reads the protagonist's expressions and can often tell when they have already done certain tasks. Though Sans is usually agreeable, he can become eerily serious to emphasize a subject. Regardless of the human being partially or wholly merciful, Sans tells them that if "the old lady" (confirmed in the True Pacifist Route to be Toriel) had not made him promise to protect them, "(they'd) be dead where (they) stand." During the Genocide Route, he does everything in his power to kill the protagonist, even faking mercy.

Sans may have a scientific background. Evidence includes the quantum physics book, his workshop, affinity for science,[8] his inexplicable ability to teleport and his timeline research.[9]

Sans has implied he once longed and searched for a life that he cannot go back to as he "gave up trying to go back a long time ago". As a Lost Soul, he tells the protagonist that "you'll never see 'em again." Although what Sans is trying to return to is vague.