Classic Sans

The Gaster Blaster is a weapon Sans uses in combat. As its name suggests, it's somehow related to V.D. Gaster. It's a fairly fast-moving, ranged weapon of varying sizes and purposes. Some fire two or more shots. The blaster resembles the skull of a dog- or dragon-like creature, firing laser beams from its mouth. When fired, the lower jaw splits in two. Sans uses it in both standard and blue modes. The beam's width directly depends on the blaster's size. During his final attack, Sans uses a large number of blasters firing in a circle. During and after each shot, the blaster's eyes begin to flicker.

Sans has the ability to use "shortcuts," which allow him to quickly cover large distances in a short time. Furthermore, Sans can appear in multiple locations within a single room simultaneously (as seen in the room leading to the mysterious door in Snowdin Forest), suggesting that Sans possesses the ability to teleport. Doggo also alludes to this, claiming that the skeleton sometimes appears nearby without any indication of approaching (this phrase can be heard if you speak to Doggo at Grillby's Pub after sharing a meal with Sans, or if you ring the bell at Doggo's post repeatedly).

Thanks to his "research," Sans is aware of timelines; Flowey notes that the skeleton is the cause of most of his resets. His knowledge of timelines makes him more observant, allowing him to detect when the protagonist is acting unusually simply by looking at their facial expression. In the Genocide Route, Sans uses the protagonist's facial expression to determine how many times they've killed them.

Sans is able to dodge the protagonist's attacks, which distinguishes him from other enemies in the game. However, his combat power is quite energy-consuming.

Sans's name is a reference to the Comic Sans font used for his dialogue. It's also a reference to the webcomic Helvetica, whose main character is named after the font of the same name. JN Wiedle, the webcomic's creator, is credited in Undertale as a special inspiration for Sans and Papyrus.

In Japanese phonetics, Sansa's name is homophonic to the Sanzu River, the boundary between death and the afterlife in Buddhist lore, which, like Sansa, judges all who cross it for their deeds.

Personality
Sans has a very laid-back personality. He enjoys sleeping, which he does during his breaks at work. He's known for his jokes, which his brother, Papyrus, greatly dislikes. 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 them.[3] He believes that actions should be motivated by a desire to protect friends and help someone, not revenge.[4]

Sans once said he doesn't like making promises, but when he does, he tries to keep them to the end. He said the only thing that saved the protagonist from death was his promise to Toriel to protect the fallen.[5] On the Genocide Route, he can no longer keep that promise, but even as he contemplates breaking it to stop a mass murderer, Sans silently apologizes to Toriel.[6]

Sans is presented as a lazy character, but it is emphasized that this laziness is a result of his knowledge of timelines, and that everything he has worked for can disappear without a trace with a single reset.[7] Furthermore, Sans himself clearly does not consider himself lazy, stating that this is as far from the truth as possible.[8]

Sans appears to have a scientific background, evidenced by his quantum physics textbook, his workshop, his love of science[9], his inexplicable ability to teleport, and his research into timelines[10].

Sans hints that he once had another life to which he was trying to return, saying 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 what exactly Sans is trying to return to.
Sans loves ketchup and drinking at Greeble's when his brother isn't looking.

Sans is a rather short and "big-boned" skeleton whose hands strongly resemble mittens. He is always seen with a wide, toothy grin on his face. Sans wears an unbuttoned blue jacket (or sweatshirt) with a gray hood and a white T-shirt underneath, black shorts with white stripes, and a pair of slippers.[2] His black eye sockets contain white pupils that disappear when he is serious or angry. When using telekinesis, Sans's left eye develops an iris, flashing blue and yellow (the colors of patience and justice), while his right pupil disappears.