Eighteen-year-old Annaleigh Thaumas is the middle daughter of the Duke of the Salann Islands. Following the sudden death of her eldest sister, Annaleigh suspects foul play and begins investigating the tragedy. She is logical and empathetic, often stepping up to mediate family disputes or care for her younger siblings. Despite the islanders' belief that her family is cursed, she remains determined to find practical answers rather than rely on superstition.

Cassius is a handsome stranger Annaleigh encounters on the fishing island. He explains he is searching for his ailing father, a sea captain he has never met. He is chivalrous, complimentary, and quickly develops a mutual attraction with Annaleigh. He frequently checks on her well-being and offers to help her process her family's tragedies.

Camille is the eldest living Thaumas sister and the designated heir to Highmoor Manor. She is desperate to escape the stigma of the family curse and secure a husband before her reputation is permanently ruined. She frequently clashes with her new stepmother and firmly believes the family should end their mourning period to embrace society events.

Verity is the six-year-old youngest sister of the Thaumas family. She suffers from intense night terrors and claims her deceased sisters visit her in the manor. Her vivid, disturbing drawings of her sisters' injuries unsettle the household, though most family members dismiss her claims as a child's overactive imagination coping with grief.

Morella is Duke Ortun's new, pregnant wife. As a mainlander, she struggles to understand the Salann Islands' customs, particularly their extended mourning periods. She eagerly encourages her husband to host balls and move past his grief, which causes tension with some of her stepdaughters, though she makes genuine attempts to bond with Annaleigh.

Duke Ortun is the ruler of the Salann Islands and the grieving father of twelve daughters. Despite losing his first wife and four children over the years, he attempts to focus on the future with his new wife, Morella. He is a loving but sometimes quick-tempered man who strictly dismisses any rumors of a family curse.

Fisher is Annaleigh's childhood friend and the apprentice to the island's lighthouse keeper. He assists the sisters with their preparations for society events and quietly harbors romantic feelings for Annaleigh. He is the first to introduce the concept of magical doors to the girls through a local fairy tale.

Eulalie is the recently deceased eldest Thaumas sister. Known for her beauty and wit, her sudden death on the cliffs is officially deemed an accident by the authorities, but her passing serves as the catalyst for Annaleigh's murder investigation.

Edgar is a local clock maker who reveals himself as Eulalie's secret suitor. He provides Annaleigh with crucial information about the night her sister died, claiming he witnessed a shadow push her from the cliffs.

Hanna is the dedicated maid at Highmoor Manor. She helped raise the Thaumas sisters following their mother's death and acts as a maternal figure in the household. She offers practical advice and comfort to the grieving family while managing the estate's staff.

Lenore is one of the sixteen-year-old Thaumas triplets. She is exceptionally close to Rosalie and Ligeia and shares their enthusiasm for dancing and society events. She relies heavily on her connection to her fellow triplets to navigate life at the manor.

Rosalie is one of the sixteen-year-old Thaumas triplets. She is energetic, outspoken, and the first of the sisters to eagerly rush into the hidden magical door they discover near the Grotto.

Ligeia is one of the sixteen-year-old Thaumas triplets. She shares her sisters' desperation to participate in balls and meet potential suitors, viewing dancing as the ultimate escape from the dreary reality of Highmoor Manor.

Mercy is one of the younger Thaumas sisters. She participates in the family's secretive nightly escapes to dance, easily swept up in the glamour and magic of the events despite her young age.

Honor is a younger Thaumas sister who fears the dark and relies heavily on her older siblings for comfort and protection. She is highly sensitive to the tense atmosphere within Highmoor Manor.

Ava is a deceased Thaumas sister who died from a plague years before the story begins. Her memory haunts the family, and she frequently appears in Verity's unsettling drawings.

Octavia is a deceased Thaumas sister who suffered a fatal fall from a library ladder, an event that heavily contributed to the local rumors of a family curse.

Elizabeth is a deceased Thaumas sister who drowned in a bathtub, leaving a lingering trauma that affects Annaleigh whenever she uses the washroom.

Cecilia is the deceased first wife of Duke Ortun and mother to the twelve Thaumas sisters. She died giving birth to Verity, a loss that fractured the family and left a void that her husband attempts to fill with his new wife.

Silas is the elderly keeper of the Old Maude lighthouse and Fisher's employer. He lives a quiet life tending to the vital safety beacon for the island's ships.

Roland is the valet at Highmoor Manor. Annaleigh briefly questions him during her investigation into Eulalie's death, though he claims he was away visiting his mother on the night in question.

Robin Briord is a local lord whom Camille hopes to court. Despite her efforts to engage him in conversation, he avoids her due to his fear of the deadly rumors surrounding the Thaumas sisters.

Sterland is an honorary uncle to the Thaumas sisters who was once engaged to Duke Ortun's missing twin sister, Evangeline. He maintains a strained but persistent connection to the family.