W. D. Gaster

W. D. Gaster is a hidden character in the world of Undertale. If the game is played normally, his existence is only hinted at by characters such as the River Person. Otherwise, Gaster is hidden throughout the game's internal data and direct references to Gaster can only be found by editing the game's files and messing around with the game's "fun" values.

Although you are not able to fight Gaster, his stats were gathered by data-mining. His enemy type also happens to be 666, yet the enemy type before that is 100 (Asriel).

Piecing the clues together reveals that Dr. Gaster was the previous royal scientist before Alphys, and that he was responsible for creating the Core. However, he fell into his creation (which is implied to be the Core but may also be the strange machine located in Sans' workshop) and his life was cut short. It is implied he was erased from existence, but that his consciousness is still present – forced to comprehend the fact that the world is the same without him.

Similar to Sans and Papyrus, W. D. Gaster speaks in a typeface different from most other characters.

Description
W. D. Gaster does not have an official known form, but there are several rooms and unused assets found in Waterfall that may point to what the royal scientist may have looked like.

Room 268 and 269
Editing the game's files so that the "Fun" value is capitalized and set to 66 may cause Room 268 (room_water_fakehallway), a mysterious hallway with a grey door, to appear between Room 94 (room_water_savepoint1) where the crystalized cheese is located and Room 95 (room_water11) where Sans' telescope is located.

The grey door leads to Room 269 (room_mysteryman) where a mysterious person (spr_mysteryman), who is widely believed to be Gaster himself, will appear. Unlike every other NPC, the person has no collision physics (they can be passed through), and interacting with them will cause them to react in surprise and vanish while a sound effect plays.

Once the player 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 you actually leave the hall. The corridor itself becomes inaccessible once the player leaves it and all values drop to 0.

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 you 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; it's unknown if it is the bug-hunting spot that Toriel talks about in the beginning of the game.

An unknown character in 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 is more likely to have been intended to be another monster, most likely Flowey (who follows you throughout the game) but it also may be 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 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 you the Annoying Dog.

Exiting this room will lead a Sound Test room, which can also be accessed by having the game's fun value capitalized and set to 65. The Sound Test does not actually contain music found in the Undertale Soundtrack but instead has four tracks titled: "Happy Town", "Meat Factory", "Trouble Dingle", and "Gaster's Theme". The latter, however, is playable.

The Stable
Inside the game's files are strings of text that implicate a stable was planned to be put into the game. One string of text says "(It's a stable for snails.) (A snable.)" which probably belongs to an unused sign in the Thundersnail room due to its position and a bunch of other lines. A bunch of other lines indicate that this is an interactable horse stable.

If the player brings a ghostly key found in a trash can, which may have been planned to be put in Napstablook's house, to the horse stable, the same ghostly figure from Room 272 appears here and says "* x" in Wingdings. This piece of evidence may prove that the ghostly figure isn't actually W. D. Gaster himself, but instead it's may have been kind of ghost horse that originally belonged to Gaster.

Main Story
W. D. Gaster does not show up in the game through normal means, but his existence is hinted at.

Wrong Number Song
In Room 70 (room_tundra_dock), the northern most part of Snowdin, the protagonist's Cell Phone may randomly get a receiving call. The phone call begins asking for somebody whose name starts with the letter "G", the caller then recognized that they have the wrong number and will then sing the Wrong Number song. This phone call is presumed to be for Gaster, but it could possibly be for Gerson, Glyde, Gyftrot or Grillby instead.

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

"The Man who Speaks in Hands"
During one of the travels with River Person, they will say to "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.

Some speculate that this could be a reference to Gaster's name since it is possible that the W. D. part of his name stands for Wing Dings, a type of font that is made up of symbols. This is very similar to how the characters Papyrus and Sans are based on fonts. The Wingdings font contains symbols for hands (which are capital letters A through I) – and supposing that Gaster is based on the font – this could possibly be a reference to him and him speaking in this font, although it is unconfirmed.

There is an unused audio file in the decompiled files of the game called 'withhands.mp3'. It seems to be a voice like that of Sans' or Papyrus' voices.

A similar line from the River Person is "Beware of the man who came from the other world." Considering its similarity to the "Man who speaks in hands" line, it might also refer to Gaster.

Monster Kid
Room 91 (room_water7) is the room that has a bunch of plaques about the monsters' tragic history and a small platform which leads to the short dock before the room where Undyne first throws spears at the protagonist. On the short dock, a grey version of Monster Kid (spr_mkid_goner) with only two spikes in their head instead of four and a slightly different shirt pattern might appear in this area.

The kid wonders about how scary it would be to imagine a world that looks exactly the same, except that they don't exist in it and how the world functions perfectly without them. You can also take an umbrella to them, triggering different dialogue. They soon prompt you to 'forget about them' and disappears when you leave the room they're in.

Some have also speculated that Gaster is actually talking through the Kid instead.

Gaster's Followers
Gaster's Followers are grey NPCs that are found near the Elevators in Hotland.

Room 169 (room_fire_elevator_l3) is the room where the protagonist leaves the elevator onto the left part of the third floor of Hotland. If the fun value is capitalized and set to 62, Gaster Follower 1 (spr_g_follower_1) will appear. This follower is a grey, emotionless version of the NPC normally found licking on the ficus in the restaurant in MTT Resort and in the unused room Room 304 (room_water_mushroom).

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. The NPC disappears once the protagonist leaves the room. Room 156 (room_fire_elevator_r1) is the room where the protagonist finds the first elevator of Hotland. If the fun value is capitalized and set to 61, Gaster Follower 2 (spr_g_follower_2) will appear. This follower is a grey version of the Donut Guy that can normally be found on the third floor of Hotland in front of the Spider Bake Sale, instead with a talking head in place of the donut.

This follower can only speak in rhymes through the head in his hand. They talk about how the old royal scientist, Doctor W. D. Gaster, suddenly vanished without a trace and was shattered across time and space. The NPC immediately disappears once they mention that they are holding a piece of him.

Room 168 (room_fire_elevator_l2) is the room where the protagonist finds the elevator to the left part of the second floor of Hotland. If the fun value is capitalized and set to 63, Gaster Follower 3 (spr_g_follower_3) will appear. This NPC is a talking face coming out of the ground and is not found anywhere else in the game.

They'll talk about how Gaster's life was cut short after an experiment went wrong. They will then stop and mention that they shouldn't gossip about someone who's listening. The NPC disappears once the protagonist leaves the room.

Sans
Sans is presumed to have a large connection with W. D. Gaster due to the workshop hidden in the basement behind Papyrus' and his house and his weapons in the Genocide Route.

To access the workshop, one must be at the end of the Neutral Route and save in the final corridor where Sans explains Execution Points and Level of Violence. The protagonist must speak to Sans repeatedly and reload their save. He will recognize your time travelling powers, and grant you a key to his room. Sans' room, after the cutscene with Papyrus when you initially first enter his room is triggered, contains a Silver Key which opens up a door behind their house.

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 a lot of people that the protagonist does not recognize. 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. Which means that the machine was never meant to be fixed and if this was the creation that Gaster fell in, then Gaster was doomed to remain erased.

Additionally, Sans uses a blaster weapon during his fight in the Genocide Route labeled "gasterblaster" indicating that the blasters are a device that was developed by Gaster. An alternate interpretation could be that it was a weapon specifically made to be used against Gaster.

When Sans falls asleep during his battle, his "Z"s are in the font "Aster" rather than Comic Sans.

Quotes
Editing the room number in your save0 and .ini file to Room 264 (room_gaster) will bring up a message that reads the following:

This quote is believed to be the seventeenth entry from the True Lab that cannot be accessed. The "two" mentioned has been speculated to be either Sans and Papyrus, Toriel and Asgore, or Sans and Alphys (since they're the only other two characters with a scientific background). The font used, Wingdings, contains hand symbols which could also be a nod to "The Man who Speaks in Hands"

Trivia

 * Attempting to name the protagonist "Gaster" will cause the game to return to the intro; however, you can edit your Undertale.ini file and change the protagonists name to "GASTER", restart the game, and it will run normally.
 * Flowey may be alluding to Gaster at the start of a Genocide Run, and again when the player travels along the ramparts of New Home. He mentions that someone is watching them. However, during the second time that he talks about this, he goes on to reference people who aren't playing the Genocide Run, but are watching videos of it instead, afraid to do the things the player is about to do.
 * W. D. Gaster is presumed to be a portmanteau of the fonts "Wingdings" and "Aster". Interestingly, while Aster is a font, it is also a suffix in botany which refers to 'denoting incomplete resemblance' between two similar plants. 'Aster' (referred to fully as Asteraceae) is also a family of flowers which includes daisies, sunflowers and buttercups. "Gast" in Swedish (plural: Gastar) is a term for a kind of ghost.
 * Gaster shares some similarities with Uboa from Yume Nikki. This 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's entry, ENTRY NUMBER SEVENTEEN, is not the only entry labeled 17. Alphys has written a 17th entry, not present in the game, that mentions monsters' inability to handle determination and speaks of them melting together and breaking down. The entry is very similar to Alphys's final remarks in the True Lab before the character exits through the elevator.
 * During the battle with Flowey, the orange soul will summon giant hands that float around. The song that plays in the background sounds like a remixed, sped up version of Gaster's Theme.
 * In the True Lab, if the protagonist does not activate the Air Conditioners and enters the area where the protagonist encounters Snowdrake's mother, there is an extremely low chance that the protagonist, while observing around the dark room, might find and describe a figure obscured by the fog in the shape of a man. It is possible that it is Gaster that the protagonist encounters.
 * In the Undertale Demo, there is an unused audio file named "grandpasemi.ogg", which is reminiscent of the first few seconds of Metal Crusher. Like Papyrus and Sans, Semi is also the name of a typeface and it can be presumed that they were planned to be the grandfather of two brothers. It is unknown there is a connection between Grandpa Semi and W. D. Gaster, but it can be theorized that Semi was Gaster's true name.