Frisk is a mystery that I can't solve. They are truly an anomaly in the timeline. Flowey in his genocide speech asks how you were resurrected, believing you to be Chara, and then says maybe Toriel took you to the ruins for a proper burial suggesting Frisk is a reanimated corpse. Then when talking to Asriel at the end of true pacifist he asks why you climbed the mountain and fell into the underground. Then there's people speculating Frisk is the good half of Chara and the two of them were the ones being talked to in Gasters number 17 entry written in Wing Ding's. This is fully up to the reader of this guide's interpretation.