Although the Peanuts character of the same name is a girl, she wears a pink two piece when they go to the beach, she is referred to as "sir" by Marcie. This is probably because of her "take charge" personality and seeming tomboyishness.

She wears a green striped shirt and black shorts most of the time and is into sports very much. She also calls Charlie Brown "chuck" when she talks to him.

Peppermint also falls asleep in class all the time. Marcie who is kind of her side kick says it's ok, she will not slip down her desk because there is a safety catch, that being her nose catching on the edge of the desk when she falls asleep.

IIrc, she also lost to Snoopy in a boxing match.

More from the official peanuts site:

Peppermint Patty is a pro on the baseball diamond, but in the classroom she's a D-minus all the way. Bold, brash and tomboyish, what she lacks in common sense she makes up for in sincerity. She's the only one who calls Charlie Brown "Chuck." Oblivious to much that goes on around her, for a long time she seemed unaware that "the funny-looking kid who plays shortstop" was a beagle. She has trouble staying awake in class; most of her waking hours in the schoolroom are spent analyzing the probability patterns of true-false tests.