Alright, for those still around, here’s a full reveal of Gaster’s backstory, as best as I can remember it:
Dr. Asani was the royal scientist before Gaster. He was a grumpy, agoraphobic man of questionable morals. He lived and breathed in his lab, and eventually he got sick, started falling down. His particular area of study was focused on the idea that magic could be encoded and described, but it was mostly a failure until near the end of his life.
He wasn’t able to accept his oncoming death, so he began to create Gaster to be a new, fresh body he could write his soul into. He (being a monster species capable of reproducing without a partner) did some pretty heinous experiments on his own infants and ended up killing one to use for a fresh empty soul. (The rest went to adoption, anonymously.) In his dying moments, he succeeded in bringing Gaster to life, but not as a perfect copy of himself, just with his knowledge. Gaster was never even truly finished, which is why his face is so simple and imperfect.
Gaster, therefore, is a monster with a real physical form and a soul attached to him through the crystals set in his palms. Asgore and Toriel discovered him and helped him develop into his own person and keep his origins secret. He turned his back on Asani’s research and focused on bettering life in the Underground. Through it, he met his followers and best friends, and developed many pieces of infrastructure including the Core.
He was a godparent to Chara and Asriel and helped raise them. It was the best time of his life. But when Chara died, everything changed. Monster society was rent in two with Asgore’s pronouncement and Toriel’s leaving. Breaking the barrier and reviving the dead kids became Gaster’s highest priority, and that turned him back to Asani’s research.
With the power of the Core, much more progress was able to be made, with determination and magical encoding. Slowly but surely, this included less ethical experimentation, until the final accident that distorted Gaster and his followers (and one of the follower’s children). An error in the magical coding scattered them all across time and space. The only partial survivor is the monster who came to be known as Riverperson, the parent of Galo, the monster known in fandom as Goner Kid. They never forgave Gaster for the incident.
Gaster, meanwhile, got scattered across the multiverse. With each shard of his soul that found another piece he appeared, temporarily, in a new timeline and got a tiny bit of his body back. His strange clothes were void, as he remembered his body but the universe didn’t. His glitch dice mechanic reflected the multiplicity of his existence. The redacted CYOA event was the bit of his soul came from Asani trying to find its way back.
Eventually, he would have gained some capacity to travel the multiverse consciously and manipulate some of the magical code at will, and I had hoped one day he’d bond to someone strongly enough to deal with what he’d done and become and settle into a new home.
