Friends (Mostly) by Barbara Joosse, illustrated by Tomaso Milian

From chapters.ca:
Henry and Ruby. Ruby and Henry. Best friends. (Most of the time.) They give the best gifts and know the best games and are the bestat keeping secrets. (Most of the time.) But even when Henry and Ruby don't get along, they know one thing: nothing is the same without your best friend. Henry and Ruby. Ruby and Henry. They belong together. All of the time.