There's Mrs. Mooney, the proprietor of the boarding house. She has a big influence on the happenings in the boarding house. Polly, her daughter, is a central figure as her actions and future are a major part of the story. Mr. Doran, a boarder, is also very important. His relationship with Polly and his reaction to the situation created by Mrs. Mooney form a large part of the narrative.
Mrs. Mooney is a main character. She runs the boarding house. Then there is Polly, her daughter. And Mr. Doran, who has a relationship with Polly, is also a main character.