In-Game References

Although not much is known about Gaster, there are certain hints and information within the game. Some of these are accessed by editing the save file and changing a value known as "FUN" (responsible for, well, fun events throughout the game) in order to trigger certain events in-game. Other information is not available in-game at all, and can only be accessed by going into the files and finding the things buried in there on your own.

Gaster Follower 1

At a fun value of 62, an NPC will appear at the Hotland elevator L3 stop. In the files, they are referred to as "spr_g_follower_1". They will talk about the old royal scientist and creator of the CORE, Doctor Gaster, whose life got cut short after he fell into his creation. They disappear once the player leaves the room.

Gaster Follower 2

At a fun value of 61, the second Gaster Follower will appear at the R1 stop. They speak in rhyme through the head in their hand and tell the player that one day Gaster disappeared and is said to have been shattered across time and space. After saying they are holding a piece of him, they immediately disappear.

Gaster Follower 3

The third Gaster Follower appears at a fun value of 63 at the L2 elevator. They talk about Gaster's irreplaceable brilliance and mention his life got cut short when his experiments went wrong. They stop talking and say they shouldn't gossip, as it's rude to talk to someone who's listening. Once the player leaves the room, they disappear and are gone.

Goner Kid

At a fun value of over 90, the player can encounter Goner Kid ("spr_mkid_goner") in Waterfall. They ask the protagonist if they have ever thought about a world where everything is the same, except they don't exist anymore and admit the thought terrifies them and tell the player to forget about them. Giving them an umbrella makes them feel better about this, despite it not raining.

The Wrong Number Song

At a fun value from 2 to 39, the player will get a phone call at the Snowdin River Person spot. The caller will say "Hello, can I speak to G..." before cutting themselves off and singing a song about how they got the wrong number. This is not directly connected to Gaster, but it can be assumed.

Naming the Human

At the beginning of the game, the player can name the first fallen human. If the player enters the name "Gaster", instead of showing a message telling the player they can't choose that name, the game immediately stops and restarts from the opening cutscene at the beginning.

Gaster's Theme

At a fun value of 65, a Sound Test menu might open in the fishing rod room. Within are 4 unused songs, including one called "Gaster's Theme" which will lock the menu until the song stops once selected, thank the player for their feedback, and then close the menu. In the files this song is simply called "mus_st_him", noticably different to the other songs which are called by their full name.

Entry Number 17

One of the unused rooms located in the True Lab is called room_gaster. It can only be accessed through save file manipulation, and will trigger text written in Wingdings. It translates to "ENTRY NUMBER 17. 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."

Sans

In a pacifist run, the player can obtain Sans' roomkey by repeatedly reloading his judgement, and, once they enter his room, find a key to the room behind his house, his lab. This room notably includes a broken machine, blueprints in illegible writing, and photos of Sans with unknown people.
During the fight against Sans in a no mercy run, he will use an attack resembling a skeletal animal head referred to as "Gaster Blasters" within the files.

The River Person

The Underground's ferryperson is the River Person, a mysterious figure whose body is never shown and that never speaks clearly. While taking their ferry, they will share cryptic messages with the player, some of which are hints about the game and help to find secrets, while others are either jokes or have a completely unclear meaning. One of these messages is "Beware the man who speaks in hands," commonly assumed to refer to Gaster.

Mystery Man

At a fun value of 66, an extra room will appear in Waterfall. This room is empty except for a single figure in the middle that the protagonist can simply walk through. If the player interacts with this figure, it will look startled for a second before disappearing.

Theories

As seen above, despite numerous hints and events possibly related to Gaster, there are barely any hard facts known about him, spurring fans' imaginations and leading to numerous theories trying to solve the mystery of W. D. Gaster. There are some things that are the same across most theories:

*Entry Number 17 was written by Gaster and W. D. stands for Wing Dings
*The Mystery Man is Gaster
*Gaster is related to Sans and Papyrus in some way
*Gaster is a skeleton

Gaster's relationship to Sans and Papyrus is the most varied across theories: sometimes he is their father, sometimes they were his experiments, and sometimes there is no personal connection at all and he was simply their boss. Sans' interest in science is assumed to have come from Gaster, so in most theories he was assisting him in some capacity.

Some theories vary on Gaster's fate after his fall, with some saying he was erased from existence entirely and was forgotten as no one except the Followers ever mentions him, others say he simply died and no one talks about him as it was a traumatic event for those that knew him. There are also some theories about the creation Gaster fell into, whether it was the DT Extractor or the machine in Sans' lab, but most commonly it is assumed to have been the CORE itself. It is unclear how exactly the Followers are linked to Gaster, but most assume they were assistants and perished with him during the accident.
Regardless of what precisely happened to Gaster, most agree that he is now in a sort of void outside of reality and can observe everything happening in the game at once and even affect it to some extent, for example the game crashing if his name is entered. Building off of him being Mystery Man, most also assume he does not have a physical body anymore and cannot interact with things in the game physically, only through the files.

Some assume the machine in Sans' lab to be something he built in order to bring back Gaster, and some of the more hopeful theories posit that Gaster might be able to return after a true pacifist ending with the help of that machine, Sans, and the human.

DELTARUNE

On Halloween 2018, Toby Fox released the demo for his new game DELTARUNE. Despite some characters from Undertale appearing in the game, it is neither a sequel nor prequel to Undertale, focuses on new characters, and is set in a different universe. However, Fox has stated that this does not mean there will be no connection between the games at all, so it is still relevant to consider information from DELTARUNE when thinking about Gaster. Since only a demo and some promotion of the game have been released so far, there is not a lot of information known, but even so there is quite a few pieces of information possibly regarding Gaster.

The Promotion

On October 30th, the Undertale twitter account suddenly changed: header and icon were replaced with simple black, and there were a number of tweets posted that were written in all caps with a similar writing style to that of Entry Number 17. Fans were told that a connection had been established, and they should await further information. One day later, on Halloween, there was one final tweet stating "SHOW YOURSELF! DELTARUNE!" with a link to what was called a survey. Upon clicking the link, that survey was downloaded and it was revealed it was actually the demo to Fox's new game, DELTARUNE.

The Game

Upon starting the game, the player gets greeted with a black screen and someone talking to them in all caps text. They state that them and the player have been connected and make the player create a vessel for themselves. Upon completion of the vessel, the first speaker gets interrupted by someone else, the vessel gets discarded, and the game starts with the player controlling the protagonist, Kris. While searching a closet at their school for chalk, Kris falls into an alternate world: the Dark World. If the player attempts to use the in-game item "Phone" while in the dark world only an error message appears and a strange sounding sound plays.

Multiple characters reference someone called The Knight, who will presumably appear later on once the full game is released. The Knight is said to know the truth about this world, that none of it is real, and shares this knowledge with characters, driving them to lose faith into the world. The phrase "Dark, darker, yet darker" is also used in reference to the Knight.

At one point in the game, there is a fun event spawning an additional room which only contains a tree. If the tree is interacted with, the text will state a man is behind it. By interacting with the back of the tree, the player can talk with the man, although he is never visible, and he will offer to give them something. If the player accepts, they will receive an egg. If, later in the game, that egg is placed into a fridge, it will still be there in future replays, even if the egg did not get received in those.

The Theories

It is almost universally accepted that the person speaking at the beginning of the game (and the person having written on twitter) are W. D. Gaster making contact with the players themselves, as he has been previously implied to be aware of the games' existence in our world. Building off of that, a lot of players assume the Knight and Gaster are also the same person, due to the phrase "dark, darker, yet darker" being related to both of them and both of them showing signs of being self-aware characters. Even the people that don't believe them to be the same person agree that there is at least some kind of connection between the two. Less widely accepted, but still a common theory is that the man behind the tree is Gaster as well, as the item he gives you is capable of existing outside of just one specific playthrough.

It is unclear still how most of these pieces fit into the puzzle, but it is a safe guess that the full version of DELTARUNE will provide more information onto the mystery of W. D. Gaster.

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