Mary Alice is the younger sister of the narrator of the story, Joey McDowdell. She accompanies Joey on their yearly visits to their eccentric grandmother each summer, and, despite her age, is a little more intuitive in understanding both their grandmother and their experiences under her care.
Mary Alice is only seven when she and her brother are sent to stay with their grandmother for the first time. Although she is initially unhappy about being sent away from her home and friends, she quickly adjusts to the new environment and her grandmother's quirky ways. She learns to appreciate before her brother that despite her outrageous actions, their grandmother has a big heart and cares deeply about the downtrodden in the community. She also is the first to voice the realization that, if they knew the truth of all that goes on during the summers at their grandmother's, their parents might not think that Grandma is a very good influence on their children. Mary Alice shows her acceptance of her grandmother's audacious personality, and the often noble but hidden motives behind her actions when she joins the older woman in conspiring to prevent an annoying neighbor, Mrs. L.J. Weidenbach, from winning all the prizes in the local talent show. Mary Alice is intelligent and capable, and her character undergoes significant growth during the progression of the narrative.
Over the years of the summers spent with their grandmother, Mary Alice grows up into a likeable, sensible, and very capable young lady. She becomes an adept ballroom dancer, and even teaches Roy Veech, the young gas station attendant in town how to be a good dance partner. Together, because of Mary Alice's confident initiative, the two young people enter the local talent contest, and win first place.