W.D Gaster is a hidden character in Undertale, known because of the FUN events and altering with the save data/files. There's very little information about him, and even his design is a mystery. Most of the drawings and speculation about him are purely headcanons and up to community interpretation. He's genuinely my favorite character due to his mystery, his known information and my own headcanons.

What we know about him from his followers that show up while altering the game's files for the FUN events is that he was the royal scientist under the King, and that his intelligence was unparalleled. He created the CORE, his number in the game is represented by 666, and he has a lot to do with the timelines and the story of the game since he fell into his own creation and was destroyed across time and space, being forgotten by everyone around him.....except for speculation that he was not forgotten by Sans.

My personal headcanon is that they worked together as lab partners, but a lot of people believe they're father and son or brothers (I personally don't because I'm crazy). W.D Gaster is most likely a skeleton because of his name being a combination of two fonts, him speaking in Wingdings during his secret events, and his relation to the Riverperson. We might get our answers on him after Deltarune is finished.