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.

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 the creation (which is implied to be the Core but may also be the strange machine located in Sans's 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 Papyrus, W. D. Gaster speaks in all capitals and in a font.

Description
W. D. Gaster does not have an official known form but there are two potential possibilities, both of forms have been encountered in Room that were planned to be in Waterfall.

Room 268 and 269
Editing the game's files so that the "Fun" value is capitalized and set to 66 will 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's 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, will appear. Interacting with the person will cause them to react in surprise and cause them to vanish while a sound effect plays.

Once the player leaves Room 269 back into Room 268, the door disappears from the corridor. The corridor itself becomes inaccessible once the player leaves it.

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 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. This encounter may be a reference to the Mystery Man seen in Room 269 who has what one would consider a "creepy smile" then disappears afterwards, but it is uncertain if this is just a different monster instead.

Room 262
Room 262 (room_water_redacted) is a dark room where Premonition plays and a figure in the room becomes more visible when walking closer towards it. Interacting with the figure will only bring up the message "
 * [REDACTED] " which implicates that a conversation was planned here but scrapped.

Attempting to check this room without altering something 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".

Main Story
W. D. Gaster does not show up in the game through normal means.

In Room 70 (room_tundra_dock), the northern most part of Snowdin, the protagonist's Cell Phone may randomly get a receiving call asking for somebody whose name begins with the letter "G" then the phone call abruptly transitions into the Wrong Number song. This phone call is presumed to be for W. D. Gaster, but it could be for Gerson, Glyde, or Gyftrot instead.

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. This could be a reference to the game show with Mettaton in which Alphys gives the protagonist hints on the answers to the questions from Mettaton, although this is very unlikely.

Some speculate that this could be a reference to his name since it is possible that the W. D. part of his name stands for Wing Dings, a type of font that includes symbols. This is very similar to how the characters Papyrus and Sans are based on fonts. The wingdings font contains symbols for hands – 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.

Monster Kid
Room 91 (room_water7) is the room that has a bunch of plaques about the monster's tragic history and a small platform which leads to the short dock before the room where Undyne first throws a bunch of spears at the protagonist. On the short dock, a grey version of Monster Kid (spr_mkid_goner) might appear.

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 him, triggering different dialog. He soon prompts you to 'forget about him' and disappears when you leave the room he's in.

It's possible that there was a timeline where the Monster Kid had died but was resurrected, presumably by Gaster, to make it possible for the protagonist to escape Undyne. 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 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.

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 on the Donut Guy that can normally be found on the third floor of Hotland in front of the Spider Bake Sale with a talking head coming out of the Donut.

This follower can only speak in rhymes through the face on the item 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 third 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 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 Sans's House and his weapons in the Genocide Route.

To access the workshop, one must be at the end of the Pacifist 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's room, after the cutscene with Papyrus when you initially first enter his room is triggered, contains a Silver Key which opens up a room 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, which later has a picture of Sans, Frisk and his friends if the True Ending was completed. The other two drawers will cause the protagonist to read the blueprint on the countertop, which is said to be written in symbols or someone's handwriting which could potentially at W. D. Gaster.

The broken machine in the room has been said to be unfixable according to Toby Fox, which means that it was never meant to be fixed and Gaster was doomed to remain erased.

Additionally, Sans uses a blaster weapon in the Genocide Route labeled "gasterblaster" indicating that the blasters are a device that was developed by Gaster.

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

"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? "

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 Sans and Papyrus, Toriel and Asgore, or Sans and Gaster himself. 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.
 * W. D. Gaster is presumed to be a portmanteau of the fonts, "Wingdings" and "Aster". Interestingly enough, while Aster is a font, it also refers to the genus of a flower.