W.D. Gaster is a mysterious figure who resided in the Underground. He only speaks in the Microsoft font “WingDings”. 
He was a royal scientist before Alphys, responsible for creating the CORE. But on day, he fell into his own creation, and basically got deleted. He floats through time and space. He is speculated to have a connection to Sans or Papyrus, possibly their guardian, due to his skeletal appearance. He has an oval head and a black robs which covers his legs. He has cracks on his face.