They weren't particularly close until Peter's family joined Anne's in hiding, but they were aware of one other's existence. Peter had heard of an event in which Anne's instructor referred to her as Mrs. Quack Quack, and he uses the moniker to mock Anne when she irritates him. I heard about you, Peter. Because you spoke so much in class, they dubbed you Mrs. Quack Quack. Peter and Anne eventually become friends, and when she needs someone to talk to, she comes to him. He calls her Mrs. Quack Quack because she talks so much.