Sunday Brennan is the protagonist of the novel. As the second-oldest Brennan sibling and the only sister, she had taken on many of the domestic responsibilities in the family, such as managing the family’s calendar, washing the dishes, and caring for their mother, Maura Brennan, while she is undergoing cancer treatment. She is also the emotional center of the family, providing support for Denny during his soccer games, for Jackie in his art, and for Shane during his anxiety attacks and meltdowns. Her brothers, father, and ex-fiancé, Kale, are all protective of her. In turn, Sunday is highly loyal to her family and has devoted her life to them. She had loved Kale for many years and wanted to start a family with him, but she could not face him after Billy’s sexual assault and her subsequent miscarriage.

Sunday transforms throughout the novel. Before she left New York, she was a shy and dutiful daughter whose shame led her to cut herself off from her family and live a quiet, solitary life as a writer in Los Angeles, showing The Negative Impact of Secrets and Lies. However, she becomes more assertive and fearless when she returns to New York.