He is W. D. Gaster (if it isnt obvious enough already), the royal scientist before Alphys and the one who made the core. Died from "falling into his creation" (Many say its the core but its not said anywhere that it was the core), What i heard is that he was meant to be an "explanation to everything" character but was scrapped thinking it would not fit with the game, Personally i say hes my favorite game character ever, and a recent patch changed abc_123_a.ogg to something being said with Gaster's dialogue sounds (The only character with more than one dialogue sound), Also latest patch made it so the grey door room can be seen without the fun value