Minnie Cooper can probably be considered the main character in William Faulkner's short story "Dry September." She is 38 or 39 years old and lives alone with her disabled mother and elderly aunt. She is a creature of habit and Faulkner introduces her by describing the way she spends nearly every day. In fact, Faulkner uses a decent amount of the story's text to provide the reader with background information on Minnie Cooper, something he does not do with any of the other characters, some of whom are not even named. This should clue you in that Minnie Cooper might be the story's main character.
It is also possible to argue that the barber is the main character. He is the central figure in most of the action of the story with alternating sections told from his point of view. However, Minnie Cooper is the catalyst for the events of the story and occupies a central role in it. In that sense, if you have to choose just one main character, I would say it would be her.