Sans (サンズ, Sans) is a major character from the indie-developed RPG Undertale. While his origins are largely a mystery, he and his brother Papyrus are some of the quirky characters that can be encountered and befriended or murdered in the Underground, the game's main setting.
Sans is a skeleton monster who lives in the frosty town of Snowdin, alongside his brother Papyrus, in a cavern populated by monsters underneath the surface of the Earth. The player character, Frisk, first encounters him after leaving the Ruins. He is seen tagging along with Papyrus as the latter attempts to capture Frisk, but consistently fails. Papyrus's schemes are half-hearted, appearing to stem from a desire for recognition and service to his fellow monsters than any sort of malice, and Sans frequently warns the player in advance and makes no effort to apprehend them; thus the player swiftly garners sympathy towards the duo. After befriending Papyrus, Frisk has the option to visit his and Sans's house.
At first, Sans appears to be a laid-back, flippant character; he has a fondness for sophomoric gags and practical jokes, and his dialogue text is presented with a typeface resembling Comic Sans MS, usually in all lowercase. However, as the game progresses, it becomes clear that Sans is more self-aware than he lets on.
In the "Neutral" route of the game, Sans is encountered in the Last Corridor late in the story. Here, Sans "judges" the player (distinct from Frisk), revealing that experience points and levels are directly linked to how willing they are to inflict pain and death on others. His speech will change depending on the course of action that the player takes throughout the game, with special dialogue if they killed Papyrus (which is notably not presented in his usual font). He also reveals that he is aware of the game's saving and loading mechanics—viewing them as a method of controlling time and space—and he encourages the player to do the right thing. At the end of this route, Sans phones the player and informs them about the end result of their actions on the monster community, dependent on which events were initiated and if any boss monsters were killed.
In the "True Pacifist" route, meaning every encountered monster is spared from defeat and befriended, Sans's soul is among those stolen by Flowey, a diabolical flower who seeks to absorb every soul for himself. The player must confront him and Papyrus, along with the rest of the boss monsters, to return their souls and free them. After Flowey is neutralized, Sans joins the rest of the now-freed monsters on the surface world.
If the "Genocide" route is taken, meaning every encountered monster and all random encounters are killed, Sans engages the player in battle as a last-ditch effort to stop them. This is significantly more difficult than the battle against the captured Sans in the "Pacifist" route. In this fight, he uses extremely swift and highly unpredictable attacks, typically utilizing "Gaster Blasters" (laser guns that resemble skulls); in addition, his attacks also inflict a rapid health draining effect refered to as "Karma", representing punishment for the torment inflicted by a Genocide run. Defensively, Sans totally avoids all of the player's attacks until the very end, where he decides to simply not attack, thus never ending his turn in the battle. To finish the fight, the player must wait for him to fall asleep and then he is defeated in a single hit. He then crawls away, dying off-screen.
In Deltarune: Chapter 1, Sans makes a cameo appearance as the player character Kris's next door neighbor, now owning and operating a grocery store.
Sans appears as a downloadable Mii Gunner costume as part of Round 3 for Ultimate, with one of his Gaster Blasters as the Mii Gunner’s arm cannon. Sakurai notes in its reveal that Sans was a popular request. Upon downloading the costume, a new remix of Sans's boss theme MEGALOVANIA is unlocked as an additional music track in the "Other" category. In contrast to most other costumes, Sans appears as a fully modeled character; his "mask" is just his head, instead of a mask with the Mii's face visible like other character-based costumes, such as Tails, Knuckles, or K.K. Slider.