Note: This profile covers a character who, as of now, is still shrouded in much mystery and subject to great amounts of speculation. This profile will attempt to only cover the little that is known about said character, noting if something is more theory than fact.
W. D. Gaster is a mysterious entity who, prior to Undertale's most recent major patch, could only be learned about in events accessed by modifying the game's save file. He was the royal scientist prior to Alphys, and is believed to be responsible for the creation of the CORE. This would make sense, as Alphys seems to be unfamiliar with the structure's layout, making it highly unlikely she had a hand in its design. Almost nothing else is known about Gaster's life aside from that and the tragic event of his death. One day, he supposedly fell into his "creation", not only ending his life, but shattering his very essence into an endless number of pieces across time and space. However, the fact Gaster ever existed at all is never brought up except by several equally as mysterious individuals. It is almost as if some part of him, or the things he accomplished in his life, are still retained in existence, while his identity itself has more faded into obscurity, or possibly vanished from the world entirely.
While it is unknown what Gaster himself looked like, a figure that is widely believed to represent him can be found in the hidden room 269 located in Waterfall. Upon entering the out of place door which leads to the room, the player will see a strange figure in its center. Unlike every other NPC in the game, the mysterious figure lacks any sort of collision physics, making it as if they almost don't even exist. Approaching the figure will cause them to turn around, noticing the player's presence and vanishing while an odd soundclip plays. After leaving the room, the door will have vanished, preventing the player from reentering. Due to the figure's skeletal appearance, coupled with the fact Gaster's name is related to the fonts Wingdings and Aster, many people have linked the character to Sans and Papyrus, both of whom were have said to simply have shown up in Snowdin one day and made themselves at home.
A second possible appearance for Gaster is located in the hidden room 272. The track "Premonition" (the same track which plays when Sans unknowingly asks Frisk about the existence of Flowey in Grillby's) plays and an amorphous, ghostly figure can be seen. The figure appears to become more and more clear as the player approaches it. Attempting to interact with it will simply bring up the message "[REDACTED]" in Wingdings, which is the font Gaster is supposed to speak in. Gaster's speech in Wingdings can also be seen in another hidden room (room 264) which is only accessible via manipulation of the save file. Accessing the room will lead to a black screen which soon begins to display Wingding text believed to be the previously inaccessible 17th journal entry from the True Lab, as well as the only one not written by Alphys. An offputting track, labelled "mus_smile" in the game files, plays over the entry. The message, when translated from Wingdings, reads as follows:
"ENTRY NUMBER SEVENTEEN
THE DARKNESS KEEPS GROWING
THE SHADOWS CUTTING DEEPER
PHOTON READINGS NEGATIVE
THIS NEXT EXPERIMENT SEEMS
WHAT DO YOU TWO THINK?"
After this, the screen will go black and the game will shut itself down. The identities of the two individuals Gaster speaks to at the end of the entry remain a mystery.
The only beings to ever make direct mention of Gaster are a number of otherworldly grey entities. The first of which is an odd, monochromatic version of Monster Kid that can potentially appear in one of the dock areas of Waterfall. Talking to the kid will result in them musing on how scary it would be to see a world that looks exactly the same, except they did not exist and yet everything continued on perfectly without them. They will then tell the protagonist to forget about them, vanishing as soon as the player leaves the room and returns. Due to Monster Kid repeatedly showing up in Waterfall along with the fact the hypothetical world the kid described seems to be what happened to Gaster, it is possible Gaster was using them to communicate, or simply decided to speak to Frisk in the form of a familiar entity.
The rest of the grey entities encountered are referred to as "Gaster's Followers", and have the possibility of appearing in various areas of Hotland. The first follower appears on Hotland's third floor, near the left elevator. They talk about how it made sense for Asgore to take his time on hiring a new royal scientist due to how impressive Gaster was. The entity then mentions how Gaster created the CORE, but his life was cut short when he fell into his creation. The follower then questions if Alphys will end up the same way. Upon leaving the room and returning, the player will find the follower has vanished.
The second follower potentially appears outside the right elevator on the first floor of Hotland. He says that while Alphys may work faster, she still can't compare to Gaster. He then goes on to mention how one day, Gaster shattered across time and space. When questioned how he knows this, he will say that he's holding a piece of Gaster in his hand, before vanishing right before Frisk's eyes. The third and final follower is a unique sprite seen nowhere else in the game. It is a giant, cyclopean face with a twisted grin that sprouts out of the ground near the left elevator on the second floor. Similar to the first follower, the face talks about how Gaster's genius was irreplaceable and how one day, his life was cut short. However, the being soon stops talking, mentioning how it needn't gossip before commenting, "It's rude to talk about someone who's listening". This seems to enforce the idea that, as the second follower mentioned, Gaster is shattered across time and space, making his presence permanent even though he no longer exists. There is also a possible indirect mention of Gaster by the strange River Person who will ferry Frisk across the river to other parts of the Underground. Every time they ride, the strange individual will say something cryptic, which is sometimes a hint and other times meaningless. However, there is a small chance the River Person will say, "Beware the man who speaks in hands". This is believed to refer to Gaster, as he is known to speak in all caps Wingdings, of which several of the letters of the alphabet are replaced with hands. The River Person can also sometimes say, "Beware the man who came from the other world". While this possibly refers to The Player, their gender is never solidified, unlike Gaster's. Why the River Person would feel the need to issue mysterious warnings about Gaster is yet to be seen.
Sans is also believed to have had some sort of relation to Gaster, one of many reasons being his hidden lab located under his house. The lab contains a set of blueprints written in strange symbols and a shrouded machine which is stated to be broken beyond repair. Checking the drawers in the room will reveal a poorly drawn picture of three people with the words "don't forget" written on it. Due to multiple hints at some sort of relationship between the two (Sans snoring in Aster font, having weapons labelled as "gasterblaster"s, etc.), it is commonly believed that the poorly drawn picture is of Sans, Papyrus, and Gaster. Considering everyone except several entities who pop in and out of reality seem to no longer remember the former royal scientist ever even existed, it is a possibility Sans keeps so that he never forgets the mysterious W. D. Gaster. Of course, whether or not this is true remains unknown.
Powers and Stats
Name: W. D. Gaster (Believed to stand for "Wing Ding Gaster")
Gender: Formerly male, now likely inapplicable as he no longer technically exists
Age: Unknown before his death, is now no longer affected by the concept of time
Classification: Monster (formerly), Former Royal Scientist, Mystery Man, Impossibility
Powers and Abilities: Acausality, Immortality (Unknown type due to his nature as both existing and not existing simultaneously), Possible Possession (Depending on whether or not he possessed Monster Kid or simply created a duplicate of them), Omnipresence, Spatial Manipulation (Created a new room between two points that were previously right next to each other, and erased it afterward), Possibly Void Manipulation (Possibly completely erased one of his followers)
Attack Potency: Unknown (The degree to which Gaster can affect reality, if he can even do so at all, is entirely unknown. However, it should be noted that attempting to name the fallen child "Gaster" results in the world being reset. This possibly means that anything too closely linked to Gaster cannot sustain its existence, as even the fallen child cannot exist should they have his name. It should also be noted that his in-game stats would technically put him beyond Flowey's God form, although if it is actually applicable is unknown)
Speed: Omnipresent (Gaster is apparently shattered across all time and space, being everywhere and nowhere as he is stuck in limbo, forced to observe a world where everything goes on without his memory. However, the subject should probably be dropped, as it's rude to talk about someone who's listening.)
Lifting Strength: Unknown
Striking Strength: Unknown
Durability: Unknown (Gaster is impossible to harm by almost any means, as he technically does not even exist, at all. Likewise, it is unlikely Gaster can really cause direct harm to anything else for the same reason)
Stamina: Unknown (This concept no longer applies to him)
Range: Unknown (Exists across all of space-time)
Intelligence: Immense in his life, as he was supposedly a genius of unparalleled intellect. Due to his omnipresence across existence, he is now likely omniscient to some degree
Weaknesses: The influence he has over reality is almost entirely unknown