Rosie is Grover's cousin in the book Two Wheels for Grover. When Grover comes to visit her and Frank for two weeks, the three cousins swim, play hide and seek, and eat ice cream.

Rosie also asks Grover to ride bikes, but every day he has another excuse as to why he cannot. Unaware that he doesn't know how to ride, Rosie goes off on bike rides alone, until Frank teaches Grover how to ride. Then the two cousins ride off to Farmer Finn's, where Grover gets his kitten, Speedy.