Arnold Hooper is the younger brother of Mr. (Harold) Hooper who came to visit Sesame Street on a number of occasions, including Episode 1404, Episode 1545, and Episode 1688. Mr. Hooper refers to Arnold as his "kid brother" since he's four years younger.
Arnold lives in Chicago with his wife Janet, their kids, and their Tante Sarah. He has some knowledge of his brother's neighbors on Sesame Street, as he procures a jar of mud from Lake Michigan for Oscar the Grouch before he makes the trip.
Like his brother, Arnold worked in his father's candy store as a child, where he learned how to make an egg cream. To their disdain, they both remember how to make it using different methods. However, they each seem to recall the same rules for playing flipping cards, as they engage in a mildly heated competition while trying to teach the neighborhood kids how to play.