Sometimes when Calvin's parents need a brake from their little son, they call their regular babysitter, Rosalyn. She is in fact also the only one who is willing to baby-sit the little "monster".
Being looked after by Rosalyn is Calvin's greatest fear. She has exactly the hold over him that his parents lack. She doesn't tolerate any disobedience, and if he is stupid enough to oppose to her rules, he is sent straight to bed. She uses this punishment no matter if it's five in the afternoon or nine in the evening. Rosalyn is the person Calvin has the most respect for.
Like all other kids he tries to take advantage of the situation when his parents are gone. He tries the best he can to convince Rosalyn that he can do what ever he wants to. However, Rosalyn is not easily fooled, so he has to figure out other ways to get his will. Sometimes he locks her out of the house, and other times he tries to blackmail her in various ways, none of the methods are ever successful in the end. Apart from sending him to bed, Rosalyn has another effective method against Calvin's behavior, his mother's phone number. When she threatens to call her up in the middle of her "date" with his father, Calvin is terrified, and behaves (for a little while...).
Because Rosalyn is a student, she has plenty of homework. She tries to combine this with her babysitting. Calvin however, is not at all interested in letting her succeed with this, and sabotages her studies the best he can.
When Calvin becomes a little to much for Rosalyn she seeks comfort from her boyfriend, through the telephone. Calvin then runs to the other phone to warn the boy about the gruesome faith he will face, if he doesn't put an end to this relationship right now!