W. D. Gaster

Dr. W. D. Gaster was the royal scientist before Alphys, and 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". His existence is only hinted by characters such as the River Person and Gaster's Followers. Similar to Sans and Papyrus, he speaks in a typeface different from other characters, and wholly in capital letters like Papyrus when deciphered into plain text.

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 himself can be found. 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 grey 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 leave 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 Papyrus (or Undyne and Papyrus) in either of these rooms, only the message "There is 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 Chara.

Room 272
Room 272 (room_water_redacted) is a dark room where "Premonition" plays and a ghostly figure in the room becomes more visible when walking closer towards it. Interacting with the figure will only bring up the message "* [redacted]" in lowercase Wingdings which implies that a conversation was planned here but scrapped.

Attempting to check this room by messing around with the save data will just show the Annoying Dog. Exiting this room will lead to a Sound Test room.

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,Gaster or Grillby instead. However, these characters are not likely due to their minimal significance in the game.

There is a known glitch with the Wrong Number Song where the avatar of whoever was spoke to last will show up as the ID of the call. Most players have reportedly received either Papyrus or Sans.

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 power room in the True Lab.


 * ENTRY NUMBER SEVENTEEN
 * DARK DARKER YET DARKER
 * THE DARKNESS KEEPS GROWING
 * THE SHADOWS CUTTING DEEPER
 * PHOTON READINGS NEGATIVE
 * THIS NEXT EXPERIMENT SEEMS
 * VERY
 * VERY
 * INTERESTING
 * WHAT DO YOU TWO THINK?
 * WHAT DO YOU TWO THINK?

Once the message is over, the screen turns black for a split 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." Although this character states many odd comments, this one in particular sticks out as it is unrelated to the game. 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, who was conceived in our meta-fiction universe.

Goner Kid
A grey version of Monster Kid (spr_mkid_goner) can appear in Room 91 (room_water7), in 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 isn’t striped, but rather 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. Goner Kid wonders about how scary it would be to imagine a world that looks exactly the same, except that they do not exist in it and how the world functions perfectly without them. The protagonist can also bring an umbrella to them, triggering different dialogue: "An umbrella...? But it's not raining. Ha, ha... You know, that does make me feel better about this. Thank you." They then prompt the protagonist to 'forget about them.' Talking to him again will prompt him to say, "Please don't think about this anymore."

Contrary to Gaster's Followers, the protagonist's phone will work if they attempt to call Papyrus while in the room.

Gaster's Followers
Gaster's Followers are grey 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 work normally 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 grey, emotionless version of the ficus Licker NPC. They have a double cross on their chest instead of four dots. They 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 grey 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.

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 resembles no other 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's 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.

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. Interacting with the drawer will cause a card to slip into the player'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 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 labeled "gasterblaster", 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.

Trivia

 * "W. D. Gaster" is presumed to be a portmanteau of the fonts "Wingdings" and "Aster."
 * 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, which includes daisies and sunflowers, as well as chamomile. All of these are like Golden Flowers, which could be (but is not likely to be) connected to Flowey. (Buttercups, while also yellow, are in family Ranunculaceae.)
 * "Aster" is a typeface, meaning that Gaster's name is made up of "Wing Ding" and "Aster." This connection is similar to how Sans and Papyrus's names are fonts. However, Wing Dings is not an Aster font.
 * "Gast" in Swedish (plural: gastar) is a term for a kind of ghost.
 * "Gast" in German (plural: Gäste) is a term for a visitor.
 * 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.
 * 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."
 * If the "Possible Gaster Sprite 1" is turned upside down, their clothing and hands somewhat resemble another face.
 * From the official tumblr of Undertale, Sans suggests doing research about Shyren’s lower body. Shyren's head and body are specifically two different beings, but an optical illusion makes them look like one entity.
 * This could be a hint for 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 has written a 17th entry, not present in the game by normal means, that mentions monsters' inability to handle determination and speaks of them melting together and breaking down. The entry is very similar to Alphys' final remarks in the True Lab before the character exits through the elevator.
 * The protagonist cannot call Papyrus while a Gaster follower is present in the room.
 * 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. It is possible that the figure the protagonist encounters is Gaster. However, it may simply be the mysterious slender Amalgamate that tucks the protagonist into bed in the True Lab.
 * The Undertale Demo contains unused information about a character named Grandpa Semi whose theme is reminiscent of the first few seconds of Metal Crusher. This character may have been planned to be the grandfather of Papyrus and Sans, knowing that Semi can refer to Semi Serif typefaces.
 * In version 1.001, this sound file returns under the name "grandpatemi" and plays when the protagonist denies feeding Temmie Flakes to Temmie.
 * Until version 1.001, all events related to Gaster were inaccessible unless a SAVE file was edited to have "fun" capitalized to "Fun".
 * The Gaster card is not included in the official Undertale tarot, which indicated that this card is either a spoiler or not canon.