"I believe in a world where humans and monsters can peacefully live together, where there is not a divide in the species you are. I believe if humans and monsters come together, then we can truly unlock the mysteries of the world and we can benefit as a whole."
"Didn't someone tell you to beware the man who speaks in hands?"
Doctor W. D. Gaster, commonly known as Gaster, was formerly the Underground's Royal Scientist who worked on several scientific experiments. When a young Sans and an infant Papyrus accidentally activated his secret Determination Extractor, he pushed them out of the way. However, the Determination Extractor fired at him and filled him with so much Determination, he was scattered across time and space.
- The Room of Super Smash Bros.: Gaster appeared and teleported behind Glitch Killer. This was likely because he was angry at glitch killer for killing other glitches.
- The Room of Geometry Dash: He secretly spied at Dark Star, while staying extremely stealthy so that nobody found out.
- The Lair of the Dark Star: He appeared to help defeat Dark Star when Sans and Papyrus were suffering the most. His first appearance in the room caused the Dark Beacon to glitch out. His name was then revealed to be W. D. Gaster.
Doctor Gaster's personality is a bit of a mystery since he never appears in the series that much. All we know is that he is normally serious and on to the point. Science is his main hobby, and he can be often seen tinkering with inventions or performing experiments. He talks quite directly, and can sometimes be rude to people as well.
However, when Gaster is with Sans and Papyrus, he acts differently. Gaster cares about his children very much, and is in a much less serious mood when around them. He likes making puns with Sans and he also loves spending time with Papyrus. However, he doesn't seem to understand Sans's and Papyrus's hijinks very well, but he tries to join their conservations. It should also be noted that he can be somewhat awkward at times, attempting to crack jokes and asking if they are funny, trying to be witty like Sans. While Gaster acts serious, there is good in his heart and he cares about others, especially Sans and Papyrus. He tries to protect others if he can if they are not a threat. He wishes to help the world with his inventions, trying to make it a better place for everyone.
- Teleportation: Gaster has the ability to teleport around as much as he pleases, like Sans. However, he isn't as subtle about it.
- Bone Materialisation: Like his children, Gaster can manifest bones of any kind, either in the air or from the ground.
- Bone Control: Said bones, should they manifest mid-air, can be controlled with the movements of his hands.
- Gaster' Hands: Gaster can summon more of his hands in combat, making them small or large. He uses his hands to crush or to hit his foes, but he can also use them to fire lasers. Some of his hands also have a color that allow him to use other abilities. The colors that have been so far are:
- Green: A green hand can generate a sphere of green energy around him and others close to him. This sphere absorbs most attacks, though it drains Gaster's energy while in use.
- Red: A red hand can fire a large red laser beam, though it takes time to charge.
- Reality Warping: Gaster can corrupt things around him, disobeying the rules of physics. However, this is a destructive power that often puts others in dangers, causing Gaster to only use it in emergencies.
- Gaster Blaster: Gaster can materialise Gaster Blasters just like his children, however his are much larger and more powerful, their left eyes glowing orange, while their right glow blue.
- While Sans's right eye glows blue, and Papyrus's left eye glows orange, Gaster's right eye glows blue while his left eye glows orange, having both colors of his kids. This seems to show that skeleton's glowing eye colors are passed down to their children.