W.D. Gaster
Undertale character
"spr_mysteryman", a sprite widely speculated to be W.D. Gaster in Undertale
First game	Undertale (2015)
Created by	Toby Fox

W.D. Gaster, or simply Gaster, is a character from the 2015 video game Undertale who was the previous "royal scientist" for the game's underground kingdom of monsters before he vanished mysteriously. He cannot normally be encountered in the game, and is never discussed directly as part of the game's main narrative. Players can only encounter events related to him either by luck or by modifying certain game files. A sprite from one of these events, internally named "Mystery Man", is generally used to portray W.D. Gaster, though the sprite's connection to the character is not confirmed.

The existence and nature of W.D. Gaster has been the subject of speculation among both fans and critics. Multiple theories exist about him, including ones that suggest that he is related to Sans and Papyrus. It has also been argued that his name is a reference to the Wingdings font, indicating that he may be a skeleton like Sans and Papyrus, who are named after the fonts Comic Sans and Papyrus.^[1]

W.D. Gaster is a character created by Toby Fox who appears in Undertale. Although he does not appear physically in the game's main story, he is alluded to through cryptic references in rare dialogue from minor characters. He is also indirectly referenced during a fight against Sans, who wields weapons that are labelled "Gaster Blasters" in the game's files.^[2] Gaster's existence in Undertale was discovered after players looked through the game's code.^[3] There are two sprites within the game's files that are believed to represent Gaster, known as "Mystery Man" and "Redacted".^{[4][better source needed]} According to IGN, the "Mystery Man" sprite appears to be based on the design of the character Uboa from Yume Nikki.^[5]

In addition to these sprites, modifying an internal code value labelled "fun" can cause hidden characters called Gaster Followers to appear in the game and divulge details about Gaster.^[4][2][6] They describe Gaster falling into "his creation" and "[shattering] across time and space", with one claiming that Gaster is still nearby and "listening".^[4][6] A Tumblr post written by Undertale creator Toby Fox, written from the perspective of Sans, also contained language suggesting that someone was "listening".^[2] Players can also modify the game to place their character in an unused room internally labelled "room_gaster", containing a message written in the Wingdings font titled "ENTRY NUMBER SEVENTEEN", in reference to a missing entry in a series of logs in Undertale's main story.^[4][2][6]

When Undertale creator Toby Fox released update version 1.001 for Undertale, fans believed that certain lines of dialogue added to the game suggested that Gaster was indeed a real character in the game.^[7] This update added a rare event that would allow the player to enter a room containing the "Mystery Man" sprite, which was originally only possible by modifying the game's code.^[6]

Despite the numerous direct references to Gaster in Undertale, the game's spiritual successor, Deltarune, which contains copious overt references to Undertale, does not contain any direct references to the name "Gaster" or a similar character as of the release of its second chapter, with the exception of an easter egg during a name-entry sequence. However, numerous pieces of circumstantial evidence suggest that W.D. Gaster is tied to the game's overarching story.^[8]

Certain elements of the game's publicity have also referenced W.D. Gaster. Prior to the release of the first chapter of Deltarune, a series of posts introducing the chapter on the official Undertale Twitter account were written in all capital letters with the account's profile information blacked out, a style similar to Entry 17.^[9] Additionally, "Royal Sciences LLC", a company founded by Toby Fox in October 2015 to manage the copyrights of both Undertale and Deltarune, appears to reference the character's job as the former royal scientist.^[10][11]

Gaster is referenced in Undertale at various points, but with only few exceptions, does not actually appear in the game. According to information found in the game, Gaster once existed in the world, but due to falling into one of his machines, caused his existence to be scattered across time and space. After this happened, no one in the world seemed to remember he existed.^[12] Through code modification or random factors, the player may encounter a character believed to be Gaster. Players can also find a theme in a secret sound test room, where the song "Gaster's Theme" could be played, and if they try to name themselves Gaster, the game will restart, not allowing it.^[2] If players name themselves Gaster in Deltarune, the game will similarly restart.^[8] While Gaster is not shown to appear in Toby Fox's other game, Deltarune, there are references to Gaster's theme at multiple times throughout the game and he is often a topic in fan theories about the game.^[13]

A rare possible event in the town of Snowdin in Undertale has the protagonist receive a mysterious phone call asking "Can I speak to G..." before the caller decides it was a wrong number and disconnects. Many fans speculated that the "G..." was intended to signify "Gaster". Undertale was localized into the Japanese language by 8-4, and this particular line was considered difficult to translate. There is no way in the Japanese syllabary to represent only the letter "G", and an early draft translation by 8-4 used が (Ga), which would have been taken as further confirmation the line referred to Gaster. Toby Fox wished for certain mysteries to remain mysteries, and the entire phone call instead remained intact in English in the Japanese version, released in 2017.^[14]

Multiple unknown characters in Deltarune have been commonly attributed to W.D Gaster. For example, there is a rare, hidden room in the Dark Worlds which the player can only access by walking between rooms. If the player reaches the room, it has a tree in which a 'man' offers them an egg; the room bears a resemblance to the shape of the room in which 'mysteryman' is encountered in Undertale. The player cannot see the man because the tree obscures him, but the narration states 'there is a man here.' When the player obtain the egg, the man seemingly vanishes and the narration says 'Well, there is not a man here.' In Chapter 2, the room contains a blue road leading to the tree instead. Due to the background music in the room being similar to Gaster's theme and the nature of a secret room which can only be encountered with a rare chance, many believe this is him.^[15]

An edition of the Undertale/Deltarune newsletter, released on Valentine's Day 2024, included many possible in-universe valentine's cards from the game's characters that fans could receive. One of which was from an unknown character who spoke in a manner evocative of W.D. Gaster, leading some to theorize him as its author. However, this is unconfirmed.^{[16][17][better source needed]}

W. D. Gaster has been a popular figure in the Undertale community, the mystery behind his existence sparking a large investigation by members of the community into the truth around him.^[12] The identity of W. D. Gaster was identified as one of gaming's biggest mysteries, by both Gaming Bolt and The Gamer staff, the latter who noted that Gaster was popular with roleplayers online.^[3][18] Kotaku writer Nathan Grayson noted that W. D. Gaster was the only secret left remaining in Undertale, discussing how, despite all of the work put into investigating Gaster, the mystery still remains unsolved, suggesting that the data in the game files could have been leftover content not intended to be found.^[2] Tunic game designer Andrew Shouldice spoke of his appreciation for "risky secrets" like Gaster that cause players to "consider a new approach that'd never crossed their mind, or reveal a submerged mechanic that they'd never interfaced with."^[19]

Gaster has been the subject of various theories, including a theory that he is a long-lost brother or father of Sans and Papyrus.^[6] Kotaku writer Heather Alexandra suggested this could be the case, citing design notes that allude to a character related to Papyrus and claiming that this could be Gaster.^[20] Players have speculated that the 'W. D.' in Gaster's name could stand for "Wingdings", which furthered the connection to Sans and Papyrus, who are named after Comic Sans and Papyrus fonts respectively; Nathan Grayson also believed that strange symbols found in Sans' lab may also be in Wingdings.^[2] Additionally, a common interpretation of the 'shattered across time and space' line suggests his existence was spread all across space and time.^[2] In reference to the NPCs found through file modification, some believe that they are fragments of Gaster speaking about the experience of existing across time and space.^[2] Writer Mattia Podini discussed the presence of techniques he suggested were used by Toby Fox to manipulate players into responding to certain details in a certain way. For Gaster, he suggested that he served as an example of players experiencing apophenia, the perception of meaningful connections between unrelated things. He argued that many things players took for granted to be true about Gaster, such as his appearance, character connections, and other background details, were not confirmed to be true.^[21] Fanbyte writer Kara Dennison speculated that Deltarune would eventually answer questions surrounding W.D. Gaster and other mysteries from Undertale.^[22]