He enjoys science fiction and loves to drink ketchup. He occasionally makes some low-brow puns and the fact that he performs at the MTT Resort hints that he may be a talented comedian. He enjoys making various strange jokes that tend to involve pranks and things that just mess with people's heads, such as 'selling fried snow' which on its own makes no sense, but then raising the price to ludicrous degrees until saying he does not want to sell it because it has 'too much sentimental value.' His pranks involve juvenile things such as eye paint on a telescope, an unscrewed ketchup bottle, and whoopee cushions. 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 becomes eerily serious at particular moments. This is because while Sans is apathetic towards life, he is never apathetic towards the people he cares about. Conversely, he suggests that this knowledge could be ' a poor excuse for being lazy.' On the other hand, when Papyrus becomes the leader in the Papyrus ending, Sans is implied to be working harder despite not denying his laziness upon being called out on it. He wonders if his indolence stems from his knowledge that any progress he makes will be erased when the timeline resets. His laziness is a combination of fatalism and apathy. Sans is laid back and flippant, often sleeping on the job and taking breaks.

When Sans uses telekinesis, his left eye flashes light blue and yellow, the colors for patience and justice, and his right pupil disappears. He has white pupils that disappear when he is serious or angered. He wears an unzipped blue hoodie, a white shirt, black shorts with white lining, and a pair of slippers.

Sans appears as a short, paunchy skeleton with a wide toothy grin, glove-like hands and large eye sockets.

Undertale sans colored sprite