Frisk was born a monster. In her time, the monsters won the war between the monsters and the humans. The monsters banished the humans to the underground. The monsters forgot how to use magic, and the humans learned how to use magic.

(Dont think so much about backstory etc, really its mostly about having Fun because overtale and Overfell have little to no almost no backstory (or at least i couldnt find any), An Overfell is like Putting Undertale Together with Underfell and Reverse the Monster and Human Part and you have Overfell with some extra spice in my case.)

Frisk: Relationship= Sans (Adoptive Father), Papyrus (Adoptive Uncle), Chara (Adoptive Sister)

(Its funny af just saying.)

Chara: Or at least somehow Similar...i know its AI