W. D. Gaster

W. D. Gaster was the royal scientist before Alphys, responsible for creating the CORE. Gaster's Followers relay different explanations as to what happened to Gaster. One iteration says that he "fell into his creation," another says that "his experiments went wrong," and the last says that he "shattered across space and time." Characters such as the River Person and Gaster's Followers hint at his existence. Similar to Sans and Papyrus, he speaks in a typeface different from other characters.

Appearance
W. D. Gaster does not have a known official form, but there are several rooms and unused assets found in Waterfall that point to what he may have looked like.

Room 268 and 269
In Room 269 (room_mysteryman), a figure widely believed to be Gaster can be found. They are black and white with a distorted, cracked face. Unlike every other NPC, they have no collision physics, and approaching them will cause them to react in surprise and vanish while a sound effect plays.

This room can be accessed when the game's "fun" value is set to 66. In this scenario, Room 268 (room_water_fakehallway), a hallway with a gray door, appears between Room 94 (room_water_savepoint1) and Room 95 (room_water11), the rooms with the crystallized cheese and Sans's telescope, respectively. The door in this hallway leads to Room 269.

Once the protagonist leaves Room 269 back into Room 268, the door disappears from the corridor, though it is possible to exit the room and trigger the door's reappearance, as the fun values do not reset until the protagonist leaves the hall. The corridor itself becomes inaccessible once the protagonist leaves it and all values drop to 0.

Upon attempting to call someone using the Cell Phone in either of these rooms, only the message "There's no response..." appears.

Room 123
Room 123 (room_water_prebird) is a removed corridor with tall grass that is presumed to connect Room 99 (room_water_bird), where the bird that carries the protagonist over a disproportionately small gap is located, and Room 116 (room_water_friendlyhub), where the six-way fork in Waterfall is. The characters in this area mention that this corridor is a bug catching spot.

An unknown character on the left side of the tall grass mentions that "there is somebody with a creepy smile behind you." The unseen character presumably disappears after the dialogue is triggered. The character with the "creepy smile" is possibly the Mystery Man seen in Room 269 which may be Gaster. However, it can also be Flowey (who follows the protagonist throughout the game), Sans, or even the fallen human.

Room 272
Room 272 is black and has a ghostly figure in the middle of it that becomes less transparent as the protagonist approaches. If the protagonist attempts a conversation with the NPC, it returns "* [redacted]*" in lowercase Wingdings, which implies that a conversation was planned here but was removed for security purposes. Exiting Room 272 leads to the Sound Test Room, where the protagonist can listen to four songs including Gaster's Theme.

All but one of the songs can be played and switched amongst each other; upon playing Gaster's Theme, the other three songs are unable to be played again even if the player tries to do so. After Gaster's Theme is concluded, the text "Thanks for your feedback! Be seeing you soon!" will be displayed and either the game will crash, or the protagonist will be put in the room with the fishing pole in Snowdin, suggesting this room was intended to be focused on Gaster.

To access this room and avoid the Annoying Dog screens, a hexadecimal editor needs to be used to change the "Dogcheck" value.

The Stable
Inside the game's files are strings of text that implicate a stable was planned to be put into the game. In said stable, there is a trash can, where a key can be found. If the key is brought to the horse stable, the same figure from Room 272 appears and says "* x" in Wingdings.

Wrong Number Song
In Room 70 (room_tundra_dock), the northernmost part of Snowdin, the protagonist's Cell Phone may randomly receive a call. The phone call begins asking for somebody whose name starts with the letter "G." The caller then recognizes that they dialed the wrong number, and proceeds to sing the Wrong Number Song. This phone call is presumed to be for Gaster, but it could be for Gerson, Glyde, Gyftrot, Grandpa Semi or Grillby instead. This event only happens when "fun" values in the game fall into a specific range.

There is a known glitch with the Wrong Number Song where the avatar of whoever the protagonist spoke to last will show up as the ID of the call.

Room 264
In Room 264 (room_gaster), the text below is displayed in uppercase Wingdings. This text is believed to be the seventeenth entry from the True Lab that cannot be accessed. Room 264 is only accessible through SAVE manipulation or debug mode, and it immediately precedes the generator room in the True Lab.

Once the message ends, the screen turns black for a second before the game closes. The audio that plays during the entry is labeled as "mus_smile" in the game files. This file can be sped up by 1000% to reveal Muffet's laugh, reversed, and then looped over.

"The Man who Speaks in Hands"
During one of the travels with River Person, they say "Beware of the man who speaks in hands." There is speculation that this alludes to Gaster since the typeface used in the entry number seventeen (room_gaster) uses hand symbols.

A similar line from the River Person is "Beware of the man who came from the other world." Considering the similarities it shares with the previous quote, it might also refer to Gaster or the monster So Sorry, whose check text reads "This creature is definitely in the wrong time and space!" as well as the file name of So Sorry's Theme being mus_wrongworld.

Goner Kid
A gray version of Monster Kid (spr_mkid_goner) can appear in Room 91 (room_water7), on a dock right before the room where Undyne first throws spears at the protagonist if the fun value is greater than 90. Like Gaster's followers, their sprites have minor differences compared to Monster Kid; their clothing has a blocky, checkerboard-like pattern, they have a bow or two horns/spines on one side, two lines behind their head, and their eyes have no pupils.

In the game files, there is dialogue of Goner Kid saying "Yo! You're a ??? too, right?". This dialogue is unused in-game but can be seen with debug mode on.

Unlike Gaster's Followers, the protagonist's phone will work if they attempt to use their cell phone while in the room.

Gaster's Followers
Gaster's Followers are gray NPCs that are found near the elevators in Hotland when the fun variable is at a certain value. They all resemble certain NPCs in the game but with a few minor differences.

If the protagonist tries to call Papyrus (or Papyrus and Undyne) while in a room with a follower, they will only get the line "The phone won't turn on," but the protagonist can still attempt to reach Toriel over the phone. The phone will normally work after leaving the room.

Gaster Follower 1
Gaster Follower 1 (spr_g_follower_1) appears outside of elevator L3 if the fun value is 62. This follower is a gray, emotionless version of the ficus licker NPC. They have a double cross on their chest instead of four dots and talk about how Gaster's life was cut short after he fell into one of his creations. They later wonder if Alphys will end up the same way as Gaster. This NPC disappears once the protagonist leaves the room.

Gaster Follower 2
Gaster Follower 2 (spr_g_follower_2) appears outside of elevator R1 if the fun value is 61. This follower is a gray version of the guy normally found in Hotland's Spider Bake Sale, with a talking head in place of the donut. This follower speaks in rhymes throughout the head in his hand. They talk about how the old royal scientist, W. D. Gaster, suddenly vanished without a trace and was shattered across time and space. They immediately disappear once they mention that they are holding a piece of him. Unlike Sans, their disappearance does not include a black screen interlude.

Gaster Follower 3
Gaster Follower 3 (spr_g_follower_3) appears outside of elevator L2 if the fun value is 63. This NPC is a talking face sprouting out of the ground and is its own character, unlike the other followers. They talk about how Gaster's life was cut short after an experiment went wrong. They will then stop and mention that they should not gossip about someone who is listening. This NPC disappears once the protagonist leaves the room.

Sans
Sans is presumed to have a connection with W. D. Gaster due to the workshop behind his house and his weapons used in the Genocide Route.

Goner Kid, the NPC that discusses alternate timelines, says "An umbrella...? But it's not raining." This relates to one of Sans's leitmotifs "It's Raining Somewhere Else" which plays on his date at MTT Resort and in his workshop. Sans also discusses his research on timelines during his battle in the Genocide Route.

On the official Undertale Tumblr, is a post that includes the line "besides, it’s rude to talk about people that are listening, right?" This quote is similar to a line from Gaster Follower 3.

Room 80 (room_tundra_sansbasement) is a workshop that has four drawers and a strange machine covered up with a curtain. One drawer contains a badge, and another contains a photo album with numerous people that the protagonist does not recognize. If the protagonist has encountered Clam Girl earlier in the route, interacting with the drawer will cause a card to slip into the protagonist's view; it displays a poorly drawn picture of 3 people, with the words "don't forget" written on it. The other two drawers will cause the protagonist to read the blueprint on the countertop, which is said to be written in symbols or handwriting that could potentially be related to W. D. Gaster.

The note written on the picture with three people is most likely from Sans; Unlike Papyrus, he does not write in his typeface, which can be seen when the protagonist reads the note on his treadmill.

In Room 264 in one of the parts of Gaster's version of Entry Number 17, he says "What do you two think?" This "two" may refer to Sans and Papyrus as they both have "font" names. An alternate interpretation to these "two" is that they consist of Sans and Alphys, as it is hinted at the end of the True Pacifist Route that they had a past together and Gaster could be the missing link to where they possibly could have had a relationship.

The broken machine in the room has been said to be unfixable according to Toby Fox. This means that the machine was never meant to be fixed, and if this was the creation Gaster fell in, then he is surely doomed to remain erased.

Additionally, Sans uses a weapon during his fight in the Genocide Route called a "Gaster Blaster," possibly indicating that the weapon was developed by Gaster. An alternate interpretation could be that it was a weapon specifically made to be used against Gaster if Gaster was kept in the game.

Trivia

 * Gaster's Theme is similar to the track "Snow Road" from the Paper Mario soundtrack. When slowed down about 20%, Gaster's Theme is almost completely identical to "Who Are You?" from Final Fantasy VII.
 * The redacted sprite in Room 272 speaks with an asterisk and in lowercase, similar to Sans. Entry Number Seventeen is written in uppercase and without an asterisk, similar to Papyrus.
 * "W. D. Gaster" is presumed to be a combination of the fonts "Wingdings" and "Aster."
 * Aster is similar to the font that Sans snores in when he falls asleep at the end of the Genocide Route.
 * While Aster is a font, it is also a suffix in botany which refers to "denoting incomplete resemblance" between two similar plants.
 * "Aster" is a genus of flowers in the Asteraceae family. This family includes daisies, sunflowers, chamomile, and other economically important species. Flowey could also be from the Asteraceae family.
 * "Gǣstan" is an Old English root word of Germanic origin that forms the root of "aghast" and "ghost." As time progressed, the root became associated with ghosts and specters as it means "terrify" or "frighten."
 * Attempting to name the fallen human "Gaster" will cause the game to return to the intro.
 * It is possible that Gaster could have created the DT Extraction Machine in the True Lab as Alphys admits to using blueprints to operate it in ENTRY NUMBER 5. This may be the blueprints found in Sans's Workshop.
 * Gaster shares some visual similarities with Uboa from Yume Nikki. This appearance is also shared with Dr. Andonuts from Toby's Earthbound Halloween Hack.
 * Gaster also shows a slight resemblance to the character Whiteface from the horror game "imscared."
 * Gaster is also similar to some versions of SlenderMan, a famous creepypasta figure.
 * Gaster is Similar to Marionette from Five Night at Freddy 2.
 * If "Possible Gaster Sprite 1" is turned upside down, their clothing and hands resemble another face.
 * From the official Tumblr of Undertale, Sans suggests doing research about Shyren’s lower body. Shyren's head and body are two different beings, but an optical illusion makes them look like one entity.
 * This could relate to the face that shows up on the body of the Mystery Man. Further interpretation could be that the Mystery Man is actually two different monsters or an Amalgamate.
 * Alphys and Gaster both wrote an Entry Number 17; neither transcript appears without file manipulation.
 * In the True Lab, if the protagonist does not activate the fans and enters the area where the protagonist encounters Snowdrake's Mother and interacts with the second refrigerator in the room they will describe a figure obscured by the fog in the shape of a man. This figure could be Gaster. However, it may simply be the "watching man" Amalgamate that tucks the protagonist into bed in the True Lab.
 * Until version 1.001, events related to Gaster were inaccessible unless a SAVE file was edited to have "fun" capitalized to "Fun."
 * "Fun" could stand for "Functional Universe Number," as Gaster was stated to have shattered across space and time. These different universes can offer novel events based on "fun" value such as the occurrence of Clam Girl or the Wrong Number Song.
 * The Gaster card is not included in the official Undertale tarot, which indicates that this card is either a spoiler or not canon.