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Assumed appearance
w.d. gaster
A dummied-out character in Undertale that can normally only be found through very rare events. Information about him is scattered around the various Fun Events, of which only one can be seen per given playthrough without hacking the random Fun Value set at the playthrough's start.
The previous royal scientist who, among other things, created the CORE and fell into "his creation," resulting in him being scattered across time and space and forgotten by almost everyone in the underground. He is also thought to be connected to Sans due to the skull-shaped beam cannons that he wields in his boss fight being named "gasterblaster" within the game's files, and is generally assumed in most speculation to be Sans and Papyrus' older brother or father.
Because any information relevant to him, or even any mention of his name and (former) existence is vague, much of the information circulated about Gaster is conjecture. His appearance is commonly speculated to be that of the mysterious figure who looks similar to Uboa appearing in the grayed-out door of the Fake Hallway event at Fun Value 66.
He is commonly theorized to be the voice speaking to the player at the start of Deltarune, and to be of major importance to its plot.
Appearance
