While I was convincing myself to not to get too attached, I figured out his backstory:
ErrorSans eventually found the Aftertale universe, and of course found GenoSans. They fought and Error won, destroying what was left of his soul.
However, Geno still wasn’t ready to die - he still had to protect Papyrus from Chara. So through his own sheer will and the determination he injected himself with, he willed himself to continue, and became the horrifying anomaly known as Fatal_Error.
So let’s recap:
Error deleted Geno from existence.
Fatal_Error, through determination, un-deleted himself and put himself back into existence.
Geno is no longer a glitch.
He’s a fucking virus.
And this is why I fell in love with this character.