As quoted from Bill Watterson:

"Probably the only person Calvin fears is his baby-sitter. I put her in a Sunday strip early on, never thinking of her as a regular character, but her intimidation of Calvin surprised me, so she's made a few appearances since. Rosalyn even seems to daunt Calvin's parents, using their desperation to get out of the house to demand advances and raises. Rosalyn's relationship with Calvin is pretty one-dimensional, so baby-sitter stories get harder and harder to write, but for a later addition to the strip, she's worked pretty well."

As quoted from Me:

Rosalyn always babysits calvin despite her best interests, the only reason she babysits him is the pay. She makes a fortune from his desperate parents. There isn't really much I can say, Bill said it all. So here's the comics: