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.
Prior to the events of Undertale, monsters and humans engage in a war, and the monsters are easily defeated. They are then sealed away beneath the surface of the Earth by a magical barrier. According to the town's locals, the skeleton monster Sans and his brother Papyrus appeared in the underground Snowdin Town one day and "asserted themselves." The player human first encounters Sans after falling into underground Ruins, then making their way out into Snowdin Forest. Sans is seen tagging along with Papyrus as the latter attempts to capture the human, but consistently fails. Papyrus's schemes, though clearly passionate, are easily foiled and appear to stem more from a desire for recognition and service to his fellow monsters than any sort of malice; Sans, meanwhile, informs the player about these schemes well in advance and only appears to participate in them to encourage his brother's pursuits. This relationship puts the duo in a sympathetic light, and after befriending Papyrus, the player has the option to visit his and Sans's house.
In terms of physical appearance, Sans is a squat, humanlike skeleton with a constant toothy smirk and apparent bags underneath his eye sockets, which have floating white pupils. He wears a light blue hoodie with a gray hood, black shorts with white lining, and white shoes (later updated to become pink slippers). He is about half the size of Papyrus. Sans is generally a laid-back, flippant character; he has a fondness for sophomoric gags and practical jokes. His dialogue text is presented with a typeface resembling Comic Sans MS, usually in all lowercase (in contrast to the standard font used for most other characters, a monospaced typeface resembling the Windows system font Fixedsys). However, as the game progresses, it becomes clear that Sans is more self-aware than he lets on.
In the majority of the game's optional story paths, Sans is encountered late in the story, right before the final boss room. Here, Sans "judges" the player, 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. He also expresses suspicion that the human has some sort of "special power," and encourages them to do the right thing. In a post-credits scene, Sans phones the human 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.
If the player avoids murdering any monsters and goes out of their way to befriend several of them, Sans's soul is among those stolen by Flowey, a diabolical, soulless flower who seeks to absorb every monster's soul for himself in order to return to his previous identity. The player must confront the lost souls of their monster friends, including Sans, to return their souls and free them. After Flowey is convinced to stop fighting, he frees the monsters from the underground, and Sans and the rest of the population return to the surface world.
If the player purposefully seeks out and murders every possible monster encounter, Sans takes on the role of the final boss of the game in place of his usual "judgement" in a last-ditch effort to stop them. In this fight, he uses extremely swift and highly unpredictable attacks, typically utilizing "Gaster Blasters" (laser guns that resemble skulls); in addition, his attacks inflict a rapid health draining effect referred to as "Karma," representing punishment for the torment inflicted by the player's actions. While Sans only has a single hit point, he consistently 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, allowing them to defeat him in a single hit. He then limps away, dying off-screen.
In Deltarune: Chapter 1, Sans makes a cameo appearance as a resident of Hometown (which is the hometown of the main protagonist, Kris), owning and operating a grocery store (which appears to be a renovated version of the "Grillby's" bar found in Undertale). Upon talking to him after returning from the Dark World, the player can ask him about his store or talk about the people of Hometown. Sans also asks Kris to come over to his house to meet his brother before the end of Chapter 1, hinting at a more significant role in future chapters.
As a costume
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, arranged by Undertale creator Toby Fox. 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.
Sans's reveal trailer holds many references to Undertale. When he is introduced, he is shown standing on Ω form PictoChat 2's textbox against a dark background, referencing the FIGHT menu in Undertale. All other stages where he is shown use pixelated graphics, referencing Undertale's use of pixel art as its main graphical medium. He dodges a Villager's up tilt (in which he uses a stick), and then a female Villager's Killing Edge swipe. These two Villagers respectively reference Frisk and Chara, along with their signature weapons, the Stick and the Real Knife. Additionally, behind the male Villager is a plate of spaghetti, likely referencing the food Papyrus comments on cooking, while behind the female Villager a hot dog can be seen, referring to the hot dog stand in Hotland run by Sans. Finally, a Fire Flower is briefly shown in front of him before a yellow Piranha Plant (which likely references Flowey, the main antagonist in Undertale, and his face mimicry ability) inherits it. He is never hit during all of his gameplay footage, as he rolls, spot dodges, and perfect shields every incoming attack. This is a reference to his unique boss mechanic in Undertale, where he would keep dodging any attack from the player throughout the battle.