Rosalyn is a teenage high school senior student and the person who watches Calvin whenever Calvin's parents go on a night out together. She is the only babysitter able to put up with Calvin's bad behavior, which she uses to demand raises and advance payment from Calvin's desperate parents. She is also, according to Watterson, the only person Calvin truly fears— certainly she is his equal in sneakiness, and is willing to play as dirty as he does. Rosalyn has a habit of sending him to bed at 6:30, which he refuses to do, and only makes more trouble. Rosalyn's boyfriend, Charlie, never appears in the strip, but calls her on the telephone sometimes. Calvin often cuts short these calls. Originally, she was created as a nameless, one-time only character with no plans to appear again; however, Watterson decided he wanted to keep her unique ability to scare Calvin, which led to many more appearances.