W.D. Gaster... the skeleton that barely anyone knows or remembers... I'm going to think of my best theory on Gaster.

W.D. Gaster was the royal scientist before Alphys. He was a skeleton and the father of Sans and Papyrus. He created the Determination extractor and spoke and wrote in a font called wing dings. Gaster also created the core.

Gaster somehow fell into his creation (the core) and was scattered across time and space, making it so no-one remembered him. The only people that do are the Gaster follower and gonner kid who you can make appear when you change the fun value on undertales settings. You can also make G appear but when you touch him, he will disappear.