I have an assumption that before Frisk went to the dungeon, she was "empty." Simply, according to my Frisk's story, terrible things happened to her as a child, which led her to an orphanage. She had no interest in anything, didn't eat, and only did what she was told to do. However, at some point, something clicked in her head, causing her to leave for Mount Ebott, where the "player" took control of her. I like the idea that after the events of the dungeon's liberation, she adopted the player's behavior and, to some extent, found herself and continued living, although she doesn't know much about her childhood other than the dungeon itself.
Oh! Interesting fact. Frisk was deaf and mute as a child, which is why she wore a hearing aid and didn't speak much. It wasn't until she was in her teens that she underwent surgery, which allowed her to hear on her own. However, she sometimes forgets words and struggles to express her feelings, leading her to use sign language.