Annie is a 25-year-old aspiring mystery writer living in London. She relies on her great-aunt's financial support to maintain her mother's home. Anxious around needles and prone to fainting at the sight of blood, she is nevertheless thrust into a high-stakes puzzle when summoned to Castle Knoll to inherit an estate. She uses journaling to process clues and relies on careful observation to evaluate the town's secretive residents.

Frances is the wealthy, eccentric owner of the Gravesdown estate whose teenage journals provide a window into the past. Following the disappearance of a childhood friend, she spends decades meticulously tracking the indiscretions of everyone in Castle Knoll. This lifelong obsession isolates her from the community, transforming her into a paranoid recluse who trusts almost no one.

Detective Crane is a police officer in his early thirties stationed in Castle Knoll. He possesses a deep familiarity with the town's residents and their intertwined histories. Though Frances frequently bothered his department with her theories, he maintains a professional, non-judgmental approach to her case.

Emily is a teenage girl from the 1960s timeline whose sudden disappearance triggers a lifelong obsession for Frances. She struggles with an emotionally abusive mother and exhibits intense jealousy toward her friends. She frequently adopts Frances's mannerisms and clothing styles while manipulating the dynamics of their social group.

Rose is a loyal member of the 1960s teenage friend group who later becomes a hotel owner in Castle Knoll. Described in her youth as having a Snow White-like appearance, she is deeply emotionally invested in her friendships. Her intense reaction to Frances's death reveals a decades-long devotion that borders on dependency.

Saxon is the local coroner in Castle Knoll and Frances's nephew by marriage. He possesses a liminal, unsettling presence and enjoys forcing others into psychological games. As a co-inheritor of the Gravesdown estate, he views the murder investigation as a competition rather than a search for justice.

Laura is a visual artist who provides a loving but financially precarious home for her daughter. She relies heavily on a monthly allowance from Frances to maintain their residence in London, remaining largely detached from the dark history of Castle Knoll.

Walter, known as Walt in his youth, is the local solicitor in Castle Knoll. He serves as Frances's long-time friend and legal advisor, responsible for executing her highly unusual will that pits the inheritors against one another.

Oliver is a property developer in London who feels little connection to his hometown of Castle Knoll. He is handsome but abrasive, eagerly anticipating the chance to bulldoze the Gravesdown estate and replace it with modern developments.

Joe is a local ambulance driver in Castle Knoll who works alongside his partner Magda. He is highly protective of his mother, Rose, and exhibits sudden bursts of aggression toward anyone he perceives as a threat to her well-being.

Lord Rutherford "Ford" Gravesdown is a wealthy young man who takes control of his family's estate following a fatal accident. He is charming, authoritative, and enjoys testing people through riddles and chess, drawing the intense fascination of the local girls.

Dr. Owusu is the practical and patient local doctor in Castle Knoll. Despite Frances's constant hypochondria and abrasive demands, the doctor maintains a genuine fondness for her and agrees to oversee her autopsy.

Archie is the head gardener for the Gravesdown estate who also runs a farm on the property. He is deeply resistant to the idea of the land being sold to developers and harbors secret activities on his farm to make ends meet.

Beth is Archie's daughter, a resident of the estate farm who cooks for Frances. She prefers vintage clothing styles and worries constantly about the fate of her family's home if Oliver Gordon succeeds in selling the property.

John is Frances's teenage boyfriend in the 1960s who eventually becomes the local vicar of Castle Knoll. He is a forgiving, decent young man whose relationship with Frances is severely tested by the social manipulation of their peers.

Magda is a purple-haired paramedic in her sixties who works the Castle Knoll route. She utilizes the mobility of her ambulance job to discreetly supply restricted items to select residents.

Elva is Saxon's wife, characterized by her snobbish attitude and open disdain for Frances. She is highly motivated by the prospect of acquiring the Gravesdown wealth and has little patience for Annie's presence.

Jenny is Annie's loyal best friend in London. She acts as a distant sounding board, helping Annie process clues and offering practical advice over the phone.

Miyuki is a large animal veterinarian married to Beth Foyle. She runs a clinic on the estate farm, managing specialized medical supplies that become relevant to the investigation.

Peter is Frances's brother. He and his wife Tansy struggle to adopt a child, a vulnerability that Emily attempts to exploit during her own chaotic personal crises.

Bill is Ford Gravesdown's personal driver in the 1960s. He provides transportation for the estate and eventually marries Rose.