Marcie gives away the biggest secret about Peppermint Patty to Charlie Brown and the world: HER REAL NAME. What is it?
Priscilla (Upon further review, the Priscilla quip was more about the Pilgrim/Charlie and Patty love story and not even a part of her real name. Ergo this and Patricia (Patty) Reinhardt will be accepted.) (edited at 11:00 am, November 24, 2017 (AEDT))
Original Answer: Priscilla