Calvin is Bebe's art assistant. He is a slow artist, which is why he mostly helps supply Bebe with her art supplies.
In Sideways Stories from Wayside School, he appears in the chapter "Bebe", where he helps Bebe with her drawings, since he is a slow artist. They create many drawings together, but by the end, learn that the quality of the art matters more than the quantity.
Also in the first book is a chapter called "Calvin", where he is asked to deliver a note to Miss Zarves on the 19th story, even though there is no Ms. Zarves, no 19th story, and no note.