Symbolism, Imagery, Allegory
Peter Schiff is the ur-dreamboat. Think of your first big childhood crush—maybe Prince Eric or Ariel from The Little Mermaid. Now add in the first school flirtation you had. Then stir in The One That Got Away. Oh, and also your version of Mr./Ms. Right.
Basically Peter Schiff is a mix of all of these. For Anne, Peter Schiff serves as a symbol of desire and love. She pins all her longings on memories of him. Her sexual awakening is bolstered by thoughts of Peter Schiff. She literally dreams about him. Even though he's a very real character in Anne's memory, she herself is aware of his symbolic heft.
And she uses Peter-as-symbol to gauge her own feelings of romance. As Peter Schiff and Peter van Daan become "one" in her head, she finally admits that she is falling in love with Peter van Daan:
Peter Schiff and Peter van Daan have melted into one Peter, who’s good and kind and whom I long for desperately. (2/28/1944.2)
Unfortunately, the Peter-Anne love connection (Petanne? Annter?) falters. But the memory of the oh-so-hunky Peter Schiff remains alive.
Peter Schiff is symbolic apart from his existence in Anne's swooning daydreams, too. The reader comes to see Peter Schiff as pointing to the horrific and senseless tragedy of Anne's death—her lust for Peter is an extension of her lust for life, and the fact that she was robbed of the chance to live and love fully is yet another symbol of the evil madness of the Holocaust.