Callowberries are a type of Elvin berry that reverse the effect of ruckleberries. They are green, orange, and speckled-looking. They are described to taste and smell like flareadon poop.
Mr. Forkle uses them to undo his disguise. Keefe threatens to sneak them into Mr. Forkle's lunch to find out who Mr. Forkle really is.
Callowberries also have medical properties.