Sophie represents the existential angst a person faces when they begin to ask philosophical questions about life. She begins the novel thinking she is real person, discovers she is fictional and living inside the mind of Albert Knag, and then has to find a way to escape Albert and become "real." She does this with the help of her mentor, Alberto, and her "double," Hilde.
As the writer of Sophie's World, Albert is a "god creator" to Sophie and Alberto. Once Sophie realizes she is fictional (or rather Albert allows Sophie to realize she is fictional), he goes on a power trip and continually finds ways to remind her how he controls her universe. This power trip bothers his daughter, Hilde, who is reading the book, so she conspires against Albert to help Sophie escape.
Hilde Møller Knag
Hilde straddles a peculiar line—she is "real" to Sophie, and yet she is fictional to the reader. Hilde is girl reading a book, but she is also a device to allow readers to infer the uncertainty of their own existence. Hilde must entertain the possibility of existing only in the mind of a higher being (who is in fact Gaarder), and the reader must entertain the possibility Gaarder (and he himself) exists only in the mind of God.
As a philosophy teacher, Alberto presumably has the self-knowledge necessary to attempt the leap out of his creator's mind, but instead, he mentors a school girl and allows her to self-actualize, too. Thus, one might argue Alberto has created the meaning in his existence that allows him to make the leap.