Walter Leddy is a boy who attended P.S. 38 around the same time as Edna Birkdale. At one point, Birkdale drew a comic of him dipping Martha's braid in his inkwell. He is a scoundrel, according to Edna.
The comic helped Nate and his scavenger hunt team (Nate's Non-Stoppers) find one of the items on the list.
Walter may be Edna Birkdale's enemy. One of his actions was notable enough for her to draw him out of disgust. However, Edna thinks that her classmate, George, is a most unpleasant brute.