There are a few other characters to mention, just for completion's sake. First is Theresa Fenneck, the librarian who witnesses Thomas chop off his own hand, and is concerned about being somehow sued by Dominick, thereby being symbolic of America's general litigiousness.
There's also Kristin, the waitress, who serves Dominick and Thomas at the beginning, and Dominick and Ray at the end. We're told that the Wequonnocs celebrate the roundness of things, and Kristin brings us full circle.
Nedra Frank, the translator of Domenico's memoir, has to be credited with the most accurate description of gramps: "Pompous, misogynistic. He's horrible, really" (2.191). Thanks, Nedra.
We also have Henry and Ruth Rood, a depressed author and his wife who keep bugging Dominick to finish painting their house. When Henry shoots himself in front of Dominick, Dominick falls off the house, ends up in the hospital, and gets reunited with Nedra Frank (whose boyfriend is Dominick's hospital roommate), an absurd chain of coincidences that sets the entire end of the novel in motion… then we never hear of the Roods again.
Finally, there are the awful Mr. LoPresto, a racist teacher, and Dell Weeks, a bully and child pornographer who often calls Thomas "Dickless" (16.21). Both of these men serve to make Ralph Drinkwater's teen and young-adult life a living hell. Thanks for nothing, dudes.