Florence can perhaps be best described as thwarted. She has suffered disappointment and frustration that she has been unable to overcome and put behind her. Her familiarity with defeat has resulted in her a deep sense of helplessness and a desire for power.
The first and most devastating blow came with the birth of Gabriel. Instantly, from Florence's view, Gabriel, their mother's favorite, was given everything that their mother could afford at the expense of Florence. When new clothes and good food were available, they went to Gabriel. The opportunity for an education went to Gabriel even though he did not want to go to school, and Florence did. Florence — powerless to make any change in her situation — watched in silent rage as Gabriel squandered away the opportunities that Florence so desperately wanted for herself.
Even as an old and dying woman, Florence is unable to overcome her dislike for her brother. She does not believe that he has changed at all, even though he claims that he was saved and has become a respected preacher. Her belief is not unfounded. She knows about his affair with Ester and the child who was a result of the liaison. Other than Gabriel, she is the only living character aware of his indiscretion, and she keeps that knowledge and Deborah's letter to herself, waiting for an opportunity to make her brother suffer for his hypocrisy.
Knowledge of Royal's existence adds fuel to the fire of Florence's disdain for her brother and her belief in his hypocrisy, and her antipathy is destroying her soul just as her illness is destroying her body. The reader, however, is left with little conviction that Florence will actually use her proof against Gabriel. For so long, events have worked against her, and she inwardly doubts her own power to bring about any change. She clings to the promise of her brother's humiliation that the letter gives her as a woman desperate for something in which to believe.
The second disappointment in Florence's life is her failed relationship with Frank. Desperate for a measure of power in her life which she had for so long been denied, Florence married Frank out of the mistaken belief that she could change (control) him. She eventually discovered that she was unable to bend Frank into the man she truly wanted. It is after she comes to this realization that he leaves her.
Throughout the novel, Florence is motivated by a search for power and a sense of control. As a young woman, she left her home and family to travel north and make her own life. As a young woman, she was strong enough to recognize that she did not want the life that awaited her in the South and to make the effort to change her fate, even though doing so meant leaving behind everything familiar to her.
After her break with Frank, Florence is able to support herself and later to help Elizabeth through her trials. Most importantly, Florence is the only character to exhibit any power over the nearly omnipotent Gabriel. In the scene following the stabbing of Roy, Florence talks back to her brother without fear. It is also she who ends the beating of Roy by grabbing the belt Gabriel uses to whip him. Ironically, Florence already has the power she seeks; she just does not recognize it. And until she does, she will remain powerless.