I would say that there are four main characters in the story Lunch Money.
Greg is the main protagonist. He's hard working and creative. It seems that he has a knack for finding ways to make money. It might be mowing lawns. It might be running a comic book business. It might be loaning money to family members with interest. He's a through and through entrepreneur, and he doesn't like competition.
Maura is Greg's first main antagonist. She has a habit of taking Greg's ideas and doing them just as well or better. In 6th grade, Greg begins his comic book selling idea, and Maura copies it. Maura and Greg end up admitting that they work better together rather than as competitors. They team up to operate a comic book selling business at school. She is every bit as tenacious and stubborn as Greg. That's probably why they butt heads at first, but end up being such good partners.
Mr. Z is Greg and Maura's teacher. He can't stand the sight of blood. It makes him woozy. Other than that, he is a strong character. He is dedicated to helping Maura and Greg work out their differences. He is the first to suggest that they are in conflict because they are so much alike. He also winds up being the only school official who takes their side in front of the school committee.
Mrs. Davenport is the school's principal. She's not really a "bad guy," but she does make a rule that students will not be allowed to sell stuff on campus. That puts a major roadblock in front of "Chunky Comics." She functions as damage control and doesn't want the possible negative aspects of the comic book business interfering with the business of education. Her stance is understandable, but the school committee eventually overrules her no comic selling stance.