Catherine is a young woman living in mid-19th-century New York City with her widowed father. She possesses a small fortune from her late mother and stands to inherit a much larger one from her father's estate. Though she lacks her mother's renowned beauty and social brilliance, she is an affectionate and deeply obedient daughter who enjoys good health. She tends to be shy in society but expresses her individuality through a love of richly colored clothing.

A brilliant and wealthy medical practitioner, Dr. Sloper enjoys a high position in New York society. He analyzes human behavior with scientific detachment, extending this clinical gaze even to his own household. He provides a comfortable life for his daughter but consistently underestimates her abilities, openly comparing her unfavorably to his late wife. He views the people around him with a sharp, often caustic wit.

Morris is a strikingly handsome and intelligent young man with a talent for social conversation. Recently returned from traveling abroad, he lacks both a formal profession and personal wealth. He resides with his widowed sister and her children. He relies heavily on his charm to secure a place in upper-class society, focusing his attention on the wealthy Washington Square household.

Lavinia Penniman is Dr. Sloper's sister, the widow of a clergyman who left her without financial means. She lives comfortably in her brother's house but possesses a highly sentimental imagination. She thrives on secrets and clandestine meetings, viewing the people around her as characters in a dramatic play she wishes to direct.

Elizabeth Almond is Dr. Sloper's sensible and prosperous sister. Married to a wealthy merchant, she manages a large, bustling household of nine children. She acts as a grounded sounding board for her brother's theories while maintaining genuine sympathy for her niece's difficult position.

Mrs. Montgomery is Morris Townsend's widowed sister. Living in difficult financial circumstances with five children, she values practicality over luxury. She provides a modest home for her brother, despite his expensive tastes and lack of steady income.

Marian is Catherine's cousin and one of Mrs. Almond's daughters. Her successful engagement party brings the extended family and their acquaintances together, setting the story's main romance in motion.

Arthur is Morris Townsend's cousin and a relation of Marian's fiancé. He helps bridge the social gap between Morris and the Sloper family, accompanying Morris on his initial parlor visits to speak with the young women.