Undertale has a ton of content that's kept fans digging through the indie RPG over the last five years. Part of what has made the game such a beloved classic is the level of detail that went into its dialogue, characters and environments, including which includes Easter eggs that are enjoyable to find. Few mysteries in recent games have been quite as enigmatic as the dark one surrounding W. D. Gaster.
Fans have uncovered information about a character called W. D. Gaster who seems to have either been cut from the game or deliberately left in the files as a secret. Dataminers also found evidence of a "fun value" that would determine when certain random events in-game would happen. Many of these event were tied to Gaster, only deepening the mystery behind Undertale's strangest character.
Because so little is known about them or their origins, Gaster has been a point of wild speculation. In fact, fans aren't even sure what Gaster looks like outside of two possible sprites. One is a shapeless ghoul with a white head and black body, which resembles the Lost Spirits from Everhood. The second possible sprite is a tall, gaunt figure with no visible features.
Most fans assume the first sprite is the actual Gaster, though even the game's files don't answer the question. The ghoul sprite is called "Mystery Man," while the featureless figure is named "Redacted." Some fans have suggested that both sprites are Gaster, pointing out there are some visible similarities between the two in terms of general body shape, as both have a distinct square-ish torso and a long head.
There's more to this character than just a sprite, as the name W.D Gaster is actually mentioned numerous times in-game by characters known as Gaster Followers. These followers are slightly altered versions of existing characters, typically devoid of color. They can be found nearby the elevators in Hotlands with the right fun value and are the main sources that tie W.D Gaster to Undertale's lore.
The first of Gaster's Followers will talk about a royal scientist before Alphys who created the CORE, which is the generator that provides the Underground with its power. Said royal scientist, Doctor Gaster, fell into his own creation. The second Gaster Follower expands on this, saying that, when Gaster fell into the CORE, he was scattered across time and space. The final follower reiterates Gaster's brilliance and claims he's listening to their conversation.
The last major piece of information about Gaster can be found in a secret True Lab entry. The room where this dialogue plays can't actually be found without modifying the game files with a save editor, though it's still an important bit of lore that helps players understand who Gaster is. This entry is written entirely in Wingdings and talks about an experiment that the writer thinks will be "very very interesting" while also describing something as being "darker, darker, yet darker." This room is called "room_gaster" in the game files.
With all of this secret content, Undertale fans have pieced together some significant information. W. D. Gaster (possibly "Wingdings" Gaster after the font) was the royal scientist who performed experiments to create a power source for the Underground. The CORE was one such experiment that would prove successful, though Gaster would fall into it and be shattered across Undertale's possible multiverse, hence why he only appears within the game's files.
There's also a fair amount of speculation on Gaster's relationship with other Undertale characters. For example, given that he's likely named after the typeface he speaks in, many speculate that he's somehow related to Papyrus and Sans. A popular theory is that Gaster was their father, especially given the strange blueprints players can find in Sans' shed near the end of the game. None of this is directly confirmed, though it certainly is possible.
Gaster has been teased to make an appearance in the upcoming sequel Deltarune through the secret True Lab entry appearing on Deltarune's website. Several references to Gaster were featured in Deltarune Chapter 1, including "Gaster's Theme" being sampled and the Entry 17 sound effect sometimes playing when the player tries to call Toriel. While none of these references have expanded on Gaster's lore, it teases that more info on Undertale's strangest secret will be found when Deltarune eventually releases.