Aunt Cathie is Papa's sister, but she appears in the book only once and never says a word. She is notable mainly for being married to Uncle Mark, who is the town's marshal, and way cool in the eyes of their three nephews.
Calvin Whitlock is the banker in Adenville and also the head of the school board, and these roles mean he is an active, though largely silent, partner in some of the book's episodes. He owns the vacant building on Main Street where Abie Glassman sets up his store, and he is the one who decides Mr. Standish must be fired for being a secret drinker—until the Great Brain confesses.
Overall, Calvin Whitlock is kind of a big deal, as we can see from his fancy backhouse. "Just by looking at the Whitlock backhouse, with its ornate scroll woodwork trim and its fancy vent, you knew Calvin Whitlock was a person of means and influence in the community" (1.4). Well, ooh la la.
Don Huddle is the blacksmith. Uncle Mark makes him deputy marshal while he and Tom search Skeleton Cave for the Jensen brothers.
Fred Harvey is the plumber. Tom's first moneymaking scheme involves charging kids a penny apiece to watch Fred Harvey dig the hole for the cesspool for the new water closet. J.D. says, "He was known as a very cranky person who didn't like kids. Mamma said the reason Mr. Harvey didn't like kids was because he had never had to put up with any of his own" (1.6). Mr. Harvey is initially annoyed by all the kids who are watching him, but as time goes by, he begins to enjoy the audience.
Miss Thatcher is the first teacher featured in the book. Though she never actually puts in an appearance, her state of health determines when the kids are in or out of school, so she significantly affects their daily lives: "Miss Thatcher was getting along in years and we kids had heard talk that she might be replaced in the fall" (2.1). She is replaced, of course, with Mr. Standish, whose new methods lead Tom and his cronies to mutiny.
Mr. Jamison is the carpenter who makes Andy Anderson's peg leg. Yep—in 1896, furniture, buildings, and medical accessories were all made by the same person.
Mr. Monaire is "the biggest sheepman in the county" (8.25). He's part of Tom's scheme to sell Lady and Brownie's puppies.
Mrs. Kay is J.D.'s friend Howard Kay's mother. She and Mamma work out between them how J.D.'s managed to get himself infected with the mumps.
Along with her son, Vassillios/Basil, Mrs. Kokovinis is a recent immigrant. She works in her husband's restaurant and does not speak English yet. The boys (and Mr. Kokovinis) all seem to think she is overprotective and wants her son to be a mama's boy, so they're determined to teach Basil to fight, even if they have to go around her to do it.