Also the lore is someone hacked and corrupted the multiverse but missed some like geno sans and then corrupted error sans attacked geno sans and made him into this
The core frisks story is that they tried to make the monsters less monstrous and the sans just punched them to the core cus they reset too much
Also this fatal error is one of the most sanest in this multiverse and because of that he became friends and even adopted core frisk