Sophie is a minor female character in Charles M. Schulz's comic strip Peanuts. Her first appearance is A Boy Named Charlie Brown and in the strip from June 18, 1968, along with Clara and Shirley, at summer camp. Peppermint Patty is her tent monitor.
Sophie appeares again in a series of strips from July 1987, and Peppermint Patty is her tent monitor again. In this series of strips, there is a running gag in which Sophie says "Here I go", and then does a flip. She does that when she wants to practice diving, or join the ballet group. Sophie appears again in strips from August 18 and August 19, 1987, in which she asks for Peppermint Patty's advice on exercise and dancing and again does flips.
Sophie is clearly younger than Peppermint Patty which is probably why she calls her, "Sir", in a misguided attempt to show respect. This habit would later become strongly associated with the character of Marcie (who strongly resembles Sophie's friend, Clara).
Behind the scenes
Charles M. Schulz's maternal grandmother was called Sophia Halverson. Sophie is most likely named for her as Schulz had an aunt named Clara and a cousin named Shirley. Sophia lived with him off and on during most of his childhood. She doted on her grandson and would help him practice hockey by playing goalie in the basement of their house. Schulz immortalized her early, albeit without a name, as the sports-loving grandmother of the children in his St. Paul Pioneer Press Li'l Folks newspaper strip, which pre-dated Peanuts. ("Wow, that's the third penalty they've given Grandma for unnecessary roughness.")