Mary Jo is a minor female character. She has never appeared in the strip but is seen in the animated TV special Someday You'll Find Her, Charlie Brown.
While he is watching TV, Charlie Brown sees Mary Jo in the audience of the Super Bowl (for two seconds which according to Charlie Brown is all the time you need to fall in love). She makes his heart melt and he is crushed when the game ends and he feels he may never see her again. Charlie Brown is determined to find her. He sets off in search of her, with the help of Linus, Snoopy and Woodstock, and they eventually get the address of the Happy Valley Farm where she lives.
At the end, however, it is revealed that Mary Jo likes Linus, and not Charlie Brown. The main reason that she likes Linus is because he has a security blanket and she has one too. She says that she has had hers since she was a baby. Mary Jo pays no attention whatsoever to the overly shy Charlie Brown, who hides behind a tree in her garden. Strangely, Charlie Brown does not seem to be very angry at Linus for stealing his potential girlfriend, even though Linus says that he and Mary Jo are going to spend the summer vacation together.
Charlie Brown acts in a different manner towards Mary Jo than he does towards other girls who he likes. Normally, when Charlie Brown likes a girl, he gets very nervous if he even just thinks about her, but with Mary Jo, Charlie Brown willingly sets out on a lengthy search to find her.
Mary Jo owns a vicious pet cat which lives on her farm.
It is also strange that Charlie Brown was not so far away when Mary Jo and Linus were talking just outside the door in fact it is possible she caught a small glimpse of him.