The four principal characters are Catherine, Dr. Sloper, Mrs. Penniman, and Morris Townsend. Dr. Sloper's sister, Mrs. Almond, and Townsend's sister, Mrs. Montgomery, are significant but secondary.
Catherine Sloper, often referred to by the narrator as "poor Catherine", is Dr. Sloper's only surviving child; her brother died at the age of three, and her mother succumbed to complications of childbirth a week after Catherine was born. She is sweet-natured and honest; however, she is also shy, plain, and not considered 'clever'. This makes her a great disappointment to her father. She has inherited an annual income of $10,000 from her late mother's estate and stands to inherit an additional $20,000, annually, from her father's. ($30,000 a year in 1880 would be equivalent to between $600,000 and $1.2 million a year in the U.S. currency of the early twenty-first century.)
Dr. Austin Sloper, a man in his early fifties, has succeeded brilliantly in his profession. He has never recovered from the death of his wife, a beautiful and vivacious woman who died shortly after Catherine's birth. Dr. Sloper is clever, experienced, perceptive, satirical, and almost always certain he is right. He often speaks ironically to Catherine who, having no way to retaliate, loves him anyway. She is also afraid of him, and defying his disapproval of Morris is a fearful step.
Lavinia Penniman, Sloper's childless, long-widowed sister, provides comic relief derived from her unrealistic romantic scheming, wild hyperbole, and duplicity. She takes a keen vicarious interest in Catherine's courtship, and later becomes infatuated with Morris as a tyrannical son, whose love affairs are of the greatest interest. She manipulates both Catherine and Morris, trying to shape their relationship into a romantic melodrama in which she plays a leading role; almost invariably, however, she makes matters worse.
Morris Townsend, a tall, handsome man of about thirty, has squandered a small inheritance traveling the world and now lives with his sister. He is a typical fortune-hunter, and Dr. Sloper immediately suspects him of this. James also gives Townsend some intelligence and grace. As a result, Catherine is unable to resist his attentions. James describes Townsend as a "statue", an "apparition", and "a knight in a poem."
Mrs. Almond, Sloper's other sister, is sensible and clever, and has a large, blooming family. Sloper frequently confides in her about Catherine's entanglement with Morris, and his sarcastic view of the situation contrasts with hers, which is more sympathetic.
Marian Almond, Mrs Almond's daughter, is an important contrast to Catherine. Marian is the conventional, flirtatious and confident young woman that Catherine isn't. She marries "punctually."
Mrs. Montgomery is a widow living in genteel poverty with her five children. Dr. Sloper pays a call on her so that they can discuss her brother, Morris. He persuades Mrs. Montgomery to admit that Morris takes money from her, returns very little, and makes her suffer.