I like the idea that Gaster created sans and Papyrus using pieces of is own soul, explaining the holes in his hands. He needed an assistant and in time a test subject to make monster stronger. sans was his assistant and become eventually a test subject to test blue magic and summon the Gaster blaster. sans was the first and afraid that he wouldn’t survive the procedure, he created papyrus, as a backup. sans quit the lab with papyrus before the core incident, (sans got daddy issues)
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After the core incident, Gaster was torn apart and turned into this pile of goo. Twisted and corrupted, he’s feeding on the determination of humans to stay strong.
blacknovelist55 