Sans

Sans (, Sans) is a major character from the indie-developed RPG. 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.

Origin
Prior to the events of Undertale, humans declared war on the monsters, easily defeating them. Monsterkind was 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 with 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 initially presented as a generally laid-back, flippant character, fond of sophomoric gags and practical jokes. However, as the game progresses, it becomes clear that Sans is more self-aware than he lets on. In contrast to Papyrus's dialogue being typeset in all uppercase, Sans's is usually typeset in all lowercase , replacing the 8-Bit Operator JVE font normally used for dialogue.

In most 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 (upon playing the game a second time) 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, Asriel Dreemurr. The player must confront the lost SOULs of their monster friends, including Sans, to return their SOULs and free them. After Asriel 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 "judgment" in a last-ditch effort to stop them. In this fight, he uses extremely swift and highly unpredictable attacks that ignore invincibility frames, typically utilizing "Gaster Blasters" (laser guns that resemble skulls) and bones; in addition, his attacks inflict a rapid health-draining effect referred to as "KR" or "Karma". While Sans only has a single point of defense and attack, 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 before forcefully moving the battle box, allowing them to attack twice in a single turn and instantly defeat him. 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 the Chapter. Sans can once again be found in Hometown after leaving the Dark World in Chapter 2, where he's running his store. Upon interacting with the leftmost shelf in the store, a cutscene triggers where Sans interacts with Kris' mother Toriel, exchanging puns before being interrupted by Asgore.

As a Mii Costume
Sans serves as the basis of the Sans Mask and Outfit DLC Mii costume for the, using one of his Gaster Blasters as the Mii Gunner's arm cannon. He is the first character from an indie game to appear as a Mii costume. Masahiro Sakurai noted upon the costume's reveal that Sans was a popular request for a fighter. Upon downloading the costume, a new remix of Sans's boss theme  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 or hat with the Mii's face visible like most other character-based costumes, such as Tails, Knuckles, or K.K. Slider. He was revealed during the  presentation on September 4, 2019.

During the reveal trailer, the Mii Gunner with the Sans outfit is first seen on the PictoChat 2 stage's Ω form with the textbox appearing against a dark background, referencing the FIGHT menu in Undertale. All other stages where he is shown in the trailer use pixelated graphics, referencing Undertale's use of as its main graphical medium. On the Balloon Fight stage, he is seen dodging a male 's up tilt (in which he uses a stick) and then a female Villager's Killing Edge swipe, referencing Frisk with the Stick and Chara with the Real Knife respectively, characters and weapons from Undertale. 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, referencing the hot dog stand in Hotland run by Sans. Next, the Mii Gunner fights a yellow with a Fire Flower, referencing Flowey, the main antagonist in Undertale. The Mii Gunner is never hit for the entirety of the trailer, as he rolls from, spot dodges, or perfect shields every incoming attack, referencing his unique mechanic in Undertale where he would keep dodging any attack from the player throughout his boss battle.

Trivia

 * Sans is the first Mii costume to come from an indie game, and would be followed by Cuphead and Shantae.
 * All three characters are also the only DLC Mii costumes to come with music tracks; Sans, however, is the only costume to come with a remix.
 * In Sans's Mii costume trailer, "Sans" is displayed on the PictoChat 2 textbox, similar to a monster's name being displayed when the ACT command is used in Undertale's menu. However, this is impossible to replicate in Ultimate; all Mii Fighters use the name "Mii" in the PictoChat 2 textbox, regardless of the name or costume used.
 * Ultimate marks Sans's first video game appearance in 3D.
 * This is also the first time Sans has been depicted with pink slippers in-game. They are depicted as white shoes in Undertale, and as either blue slippers or sneakers in official art. The pink slippers were a concept created by the Undertale fanbase and eventually adopted by official merchandise. A Sans cosmetic in  released in November 2023 as part of a collaboration with Undertale and other indie titles would also feature pink slippers.
 * The North American version of the news article for the Sans Mii costume references a recurring phrase from Undertale: "It fills you with determination."
 * Initially, all European regions instead used text noticeably written from Sans's perspective, including his trademark all-lowercase text (with the exception of specifically formatted names such as "Mii"). It has since been changed for all regions to match the North American text more closely.