Kate is an approximately 24-year-old woman born into an aristocratic family that has fallen into ruin. After her mother's death, she lives in her Aunt Maud's wealthy home. She values her independence but finds herself drawn to the comfort and security of her aunt's wealth. She feels pressure from her family to marry a rich man, though she is secretly engaged to a penniless newspaper writer.

Milly is a young, wealthy, orphaned New York heiress traveling through Europe. She possesses an immense fortune but suffers from a grave, unnamed illness that dictates her desire to live life fully while she can. Lacking immediate family, she approaches the social circles of London with curiosity and a forgiving, gentle spirit.

Merton is a newspaper writer working in London. Though he lacks an aristocratic title or significant wealth, he maintains the appearance of a gentleman of leisure. He is deeply in love with Kate Croy but struggles with his inability to provide the financial security her family demands.

Susan is a middle-aged widow from New England who writes local color stories for magazines. She meets Milly in Boston and eagerly agrees to accompany the young heiress to Europe. She is deeply protective of Milly and attempts to shield her from emotional and physical pain.

Maud is a wealthy, formidable widow who uses her immense fortune to impose her will on her social circle. She takes in her niece, Kate, with the strict intention of marrying her off to a titled aristocrat. She approaches social interactions with a calculating, business-like mindset.

Lord Mark is a titled aristocrat who lacks significant personal wealth or a clear professional direction. He moves comfortably through London's elite social circles and seeks a profitable marriage. He often communicates uncomfortable truths about the transactional nature of their society.

Lionel is Kate's father, a man who has committed an unnamed terrible deed that resulted in his family's financial and social ruin. He demands that Kate remain with her wealthy aunt, hoping to eventually profit from her potential inheritance.

Marian is Kate's widowed sister, living in precarious financial circumstances with four children. She pressures Kate to follow their aunt's instructions and marry a wealthy man, expecting Kate to support her financially in the future.

Sir Luke is an eminent London physician whom Milly consults privately about her failing health. He recognizes the severity of her condition and advises her to live her life as fully as possible.

Eugenio is a local fixer and servant in Venice who manages the opulent palazzo Milly rents. He handles the practical arrangements of her stay and observes the dynamics of her guests.