You guys never saw this coming!
Frisk fell with a baby girl. While unharmed in the fall, the child would never see Frisk again, even through her adventure. Frisk took the child through the ruins, and into the Underground. He then gave the child to Sans and Papyrus, telling them to raise her in the ruins with their friends. Ten years later, Frisk died at Asgore's hands, but his soul was depleted from determination. They left the child alone, being Undyne's favorite fighter. Asgore saw the child as a daughter, her will to lead was impeccable and her spirit in a fight was unmatchable. During her ten years, she made many friends, and many enemies followed.