Notes

Sans is shown to be very laidback, sleeping on the job and taking breaks often. He enjoys making bad, skeleton-related puns; although the fact that he performs at MTT (mettaton) Resort hints that he may, in fact, be a talented comedian. He likes science fiction and loves to drink ketchup. In addition, he hates making promises.

He is kind and reassuring but becomes eerily serious at particular moments, sometimes causing his pupils to disappear leaving empty black sockets. He is also very observant; his lines and actions change in relation to the protagonist's choices more than any other character's.

Sans's laziness is shown to be in fact apathy and fatalism. He wonders if his indolence stems from his knowledge of the fact that any progress he makes will be erased abruptly as soon as the timeline resets, though he dismisses it as an excuse for being lazy.

Judging by the quantum physics book, workshop, and broken machine found in his house; among other evidence, Sans is heavily implied to come from a scientific background.
Sans seems to know if the protagonist has achieved an impossible ending in the Neutral Route's phone call and will call them a "dirty hacker". Given that this also counts as an error handling message, he is aware of when the game's code suffers bugs as well.

Sans is also shown to have some skill in playing the trombone, though he is only seen playing it once. Papyrus finds his "incidental music" annoying, so he must play it on other occasions. Sans then tells the protagonist that Papyrus has mentioned a flower has been talking to him, and mistakenly believes that someone is tricking him with an Echo Flower when in reality the flower in question is Flowey.

Sans is later found outside of MTT Resort, where he offers the protagonist to grab a bite at its restaurant. If they agree, he will escort them through one of his 'shortcuts'. At the restaurant, Sans reveals he had befriended a woman with a similar affinity for puns that resided on the other side of the entrance to the Ruins. He tells the protagonist that the woman made a request to which he promised to comply: to watch over and protect any human who came through the door. He states that if not for his promise, the protagonist would be "dead where [they] stand", which he dismisses as a joke shortly afterwards. He mentions how he's doing a good job at protecting the protagonist, though he will question it if they have died before. Before leaving, he tells the protagonist to take care of themselves, because there's someone who really cares about them.
Sans cares for and admires his brother. He reads him bedtime stories and gives him presents under the identity of Santa. Nonetheless, he likes to mess with him as he would with anyone else.

Sans is worried about others hurting Papyrus or taking advantage of him. If the protagonist kills Papyrus, Sans will resent them greatly and disappear for most of the game. On various endings, Sans is afraid to tell Papyrus about other characters' deaths, so he lies about them going on a vacation.
Toriel and Sans are close friends, sharing a love of bad jokes; though they never see each other in person until the end of the True Pacifist Route, the Family Ending, or the Exiled Queen Ending.

While not being one to keep promises, Sans complies with her request to not harm any human that passes through the door exiting the Ruins. At the end of the Genocide Route, he indirectly apologizes to her, as he is forced to break his promise to stop the protagonist.