Sans appears as a relatively short and paunchy skeleton, with mitten-like hands. He has white dots for pupils in his black eye sockets that disappear when he is serious or angered. When Sans uses telekinesis, his left eye flashes light blue and yellow, the colors for patience and justice, and his right pupil vanishes. No matter the mood Sans is in, he is always seen with a wide, toothy-grin on his face. His clothes consists of a blue hoodie with a gray hood, a white shirt underneath, seemingly a turtleneck as seen in his battle sprite, black shorts with white stripes on the sides, and a pair of slippers usually depicted as white or pink. Sans is lazy and relatively laid-back. Sans's dialogue reflects his personality, as he speaks in all lowercase, not even capitalizing names or sentence starters, with a few exceptions, such as the initial beginning of his judgments and other more typical changes to his style of speaking for the sake of emphasis. His laziness is a combination of fatalism and apathy due to being self-aware and knowing of timeline resets. Conversely, he suggests that this knowledge could be "a poor excuse for being lazy." Sans enjoys being mischievous, primarily acting this way towards those he is on good terms with. His antics range from an assortment of illogical, juvenile, and slightly inconvenient pranks; examples include 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. 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. 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 dislikes making promises, he respects Toriel's request to watch out for any humans. Even if the protagonist kills people that were close to him, Sans refrains from broaching the topic until the Last Corridor. In Neutral Route endings where Papyrus or Toriel are alive, Sans does not tell them that the protagonist is responsible for killing monsters. Sans is also observant; he can often tell that the protagonist has already done certain tasks, such as SAVING and reloading, by reading their expressions. Though Sans is usually agreeable, he can become serious to emphasize a subject. On the Genocide Route, he does everything in his power to kill the protagonist, even feigning mercy. It is implied that this is because he knows the consequences should the protagonist succeed. Sans may have a scientific background. Evidence includes the quantum physics book, his workshop, affinity for science fiction, his ability to teleport and taking part in timeline research at one point. He is also actively employed as a sentry by the Royal Guard, citing the pay as a reason, although it's unknown who hired him. 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 during the battle with Asriel, he tells the protagonist that "you'll never see 'em again." However, as Sans's statements in this case are vague and nonspecific, it is currently unknown what he refers to. He can use gaster blasters which are dragon-like skulls that shoots beams of concentrated magic (white). He can only use 10 at once, and he can't fire them at the same time. He can use femur-like projectiles and psychokinesis to make the opponent levitate and throw them in any direction.

Sans' older brother. Energetic and caring for Sans. He loves cooking spaghetti even if he fails mostly. He calls himself "The great papyrus" and speaks in all caps. Can only use bone attacks and can turn the soul blue keeping someone grounded (unable to jump or leave the ground). Can use blue bone attacks (Only damages if you're moving)