Core also known as Frisk is a result from doing many genocide runs that she glitched making herself as a grey access timelines
Error also known as Sans is a mentally insane after he suffered so many Genocide runs and ejected himself with DT to be on the players load screen to use the reset button but was left there for so long he was formerly known as Geno until he escaped into anti-void
Error lost his brother papyrus and emotion but he is Angered very easy and is known as destroyer of the AU