Sans

Sans is a rather low and "big—boned" skeleton whose hands very much resemble mittens. He can always be seen with a wide toothy smile on his face. Sans wears an unbuttoned blue jacket (or hoodie) with a gray hood and a white T-shirt underneath, black shorts with white stripes and a pair of slippers. White pupils can be seen in his black eye sockets, which disappear when he is serious or angry. During the use of telekinesis, an iris appears in Sans's left eye, flashing blue and yellow (the colors of patience and justice), while the right pupil disappears.

Sans is a very laid—back person. He likes to sleep, which he does during breaks at work. He is known for his jokes, which his Papyrus brother dislikes very much. Sans is also kind and friendly. He believes in the good in others, and that they do bad things because something in their lives led to it. He believes that the reason for any action should be the desire to protect friends and help someone, not revenge.
Sans once said that he did not like to make promises, but if he did, he tried to keep it to the end. According to him, the only thing that saved the protagonist from death was that he promised Toriel that he would protect the fallen people. On the path of Genocide, he can no longer keep this promise, but even as he is about to break it to stop the mass murderer, Sans mentally apologizes to Toriel.
Sans is presented as a lazy character, but it is emphasized that this laziness is the result of his knowledge of time lines, that everything he worked on can disappear without a trace with just a reset. Besides, Sans himself definitely does not consider himself lazy, stating that this is as far from the truth as possible.
Apparently, Sans has a scientific background. As evidence, one can cite a textbook on quantum physics, his workshop, his love of science, his inexplicable ability to teleport and research in the field of time lines.
Sans hints that he once had another life that he tried to return to, saying that he "gave up trying to go back a long time ago." When Sans becomes a lost soul, he tells the protagonist: "you'll never see them again." Although it's unclear exactly what Sans is trying to get back to.

Sans's attitude towards the protagonist depends on the latter's actions, but usually Sans likes to joke around with him and hang out with him from time to time if the protagonist spared Papyrus. At the end of the Neutral Path and the path of Genocide, he judges the protagonist by his EXP. At the end of the True Pacifist Path, Sans considers him a good friend. If the protagonist kills Papyrus, Sans will not meet with him until the trial in the Last Corridor, where he will remind the protagonist of the murder of his brother.
Sans considers the protagonist an "anomaly"; his intentions to befriend the protagonist were in the hope that he would stop resetting the timeline. However, Sans also mentions that the protagonist did not die at their first meeting because of the promise he made to Toriel. This can be understood in different ways: he could kill the protagonist, protect him from mortal danger, or just make a joke.
His awareness of timelines makes him scrupulous and allows him to recognize when the protagonist is behaving unusually. By the look on the protagonist's face, Sans can determine whether the protagonist has experienced certain events before and whether he repeats them again. He also knows how much EXP the protagonist has, commenting on it during the trial.
If the protagonist adheres to pacifism, Sans loses his former apathy and sincerely supports the protagonist in his mission, as he says during the meeting at the MTT resort and during the trial in the Last Corridor.