Ashlyn is a rare book dealer and the owner of An Unlikely Story. She possesses psychometric abilities, which allow her to read the emotional vibrations left on old books. Having lost both her parents to tragic circumstances during her youth, she maintains a guarded personality and finds her greatest sense of safety in literature. She begins researching the authors of two mysterious, handbound books that emit intense feelings of heartbreak.

Marian is the young daughter of a wealthy New York businessman. She feels trapped by her father's authoritarian rule and strongly dislikes her arranged engagement to a man she does not love. Unlike her older sister, she rejects the rigid expectations placed upon women in her social circle. Her life changes rapidly when she meets an English writer at a party and pursues a risky romance.

Hugh is an English aspiring novelist who travels to the United States on assignment during the early days of World War II. He works for a newspaper and plans to write an exposé on a powerful businessman with fascist ties. His professional objectivity shatters when he falls in love with his subject's daughter. He carries a strong sense of social justice shaped by his working-class upbringing.

Ethan is a political science professor at the University of New Hampshire and a nonfiction author. After the death of his father, he clears out the family home and donates boxes of old texts to a local shop. He initially shows little interest in exploring his ancestors' history but soon joins Ashlyn in her investigation, drawn both to the mystery and to Ashlyn herself.

Martin is a ruthless New York businessman who built his wealth through illegal alcohol smuggling before transitioning to legitimate enterprises. He exercises tyrannical control over his household, demanding perfect obedience from his daughters. He actively aligns himself with pro-Nazi, isolationist organizations in the years before America enters the war.

Corinne is Marian's older sister. She took over the household duties at age seventeen after their mother was sent to a sanitarium. She desperately seeks her father's approval and strictly enforces his rules within the home. She resents Marian's rebellious nature and believes her sister should accept her assigned duties without complaint.

Helene is the late wife of Martin Manning. A French woman of Jewish descent, she suffered from severe depression and isolation in her marriage. After suffering a breakdown during a dinner party, her husband committed her to a sanitarium, where she later died. Her memory deeply influences her younger daughter's life choices.

Geraldine, known professionally as Goldie, is an heiress who inherited a coal fortune and a string of newspapers. She uses her publications to campaign against fascism and champion progressive social causes. She maintains a reputation as a party girl but proves to be a shrewd, fiercely protective employer who demands loyalty from her writers.

Teddy is an eligible bachelor from a highly prestigious New York family. He shares the isolationist and antisemitic views common among his social class. He views his impending marriage as a beneficial merger of family fortunes rather than a romantic partnership.

Richard, often called Dickey in his youth, is Corinne's son and Ethan's father. As a young man, he frequently runs errands and delivers discreet messages for his family members. He later breaks from his mother's strict expectations by marrying a woman she disapproves of, creating a permanent rift in the family.

Frank is the late founder of the rare books store. He recognized Ashlyn's unique psychometric abilities when she was a teenager and validated her experiences. Following the tragic loss of her parents, he offered her employment, an apartment, and a stable sense of family.

Daniel is Ashlyn's deceased ex-husband. He worked as a teaching assistant when they met and later struggled as an aspiring writer. He frequently blamed his professional failures on his wife and engaged in multiple affairs before his sudden, traumatic death on a city street.

Catherine is the late wife of Richard Hillard and the mother of Ethan. Her marriage to Richard caused significant tension within his family, though she found an unexpected ally in his aunt Marian.

Zachary is Marian's son, raised on the West Coast. He shows an early aptitude for music and attends the Berklee College of Music. He grows up to become a highly successful professional violinist playing for the Chicago Symphony Orchestra.

Ilese is Marian's daughter. Raised in California, she pursues an academic career and becomes a professor of women's studies in New York. She marries and raises three daughters, maintaining close ties with her childhood family.

Steven is an aspiring novelist and journalist in 1940s New York. He lives and works closely with the newspaper publisher Geraldine Spencer. He eventually authors a highly critical article that drastically alters the Manning family's public standing.

Ruth is a public librarian who frequently assists Ashlyn with historical research. She uses her archival access to track down old articles, obituaries, and photographs related to the figures mentioned in the companion books.

Kevin is the owner of a local vintage boutique. When he receives donations of items that fall outside his usual inventory, he passes rare or interesting books along to Ashlyn for her specialized shop.