Nancy is an 18-year-old amateur detective who lives with her father in River Heights. She drives her own convertible and travels independently, investigating crimes with a methodical approach that sets her apart from others her age. When she is called to investigate a haunted house and simultaneously deals with threats against her father's legal practice, she treats every strange phenomenon as a solvable puzzle.

Gomber is an agent representing landowners in a dispute against a railroad company. He presents himself as a respectable businessman with a sentimental desire to buy the historic Twin Elms estate. His overly polite demeanor masks a manipulative nature, as he frequently changes his tactics depending on his audience to achieve his financial goals.

Helen is Nancy's close friend who initially reacts to the strange occurrences at Twin Elms with genuine panic. Despite her fear, she proves to be a loyal assistant during the investigation. She frequently helps Nancy with experiments, searches the mansion grounds, and participates in late-night stakeouts.

Carson is a prominent lawyer and a mentor figure who trusts Nancy's judgment completely. His legal work for a railroad company places him in direct opposition to local landowners holding out for a better settlement. He travels to Chicago to track down a missing signature, pulling him directly into the criminal conspiracy.

Willie Wharton is a local landowner holding out for more money from the railroad company. His missing signature on a land-sale contract creates the central legal dispute that Carson Drew is attempting to resolve. He relies heavily on Gomber to handle his affairs.

Miss Flora is the gentle-hearted owner of the Twin Elms mansion who is over eighty years old. She is deeply attached to her ancestral home and refuses to sell it despite her growing terror of the supernatural events occurring there. The stress of the apparent haunting takes a severe toll on her physical health.

Aunt Rosemary is Miss Flora's pragmatic daughter. She acts as a protective buffer for her mother and welcomes Nancy's involvement in the case. She suspects there might be a rational explanation for the strange occurrences in the house but struggles to manage her own anxiety as the events escalate.

Hannah is the Drew family's loyal housekeeper and acts as a maternal figure to Nancy following the death of Nancy's mother. She provides emotional stability during the investigation and handles crucial long-distance communications. She never attempts to limit Nancy's detective work.

Harry is a local taxi driver working at the Cliffwood train station. He becomes caught up in the criminal plot when he drives Carson Drew from the station. He initially keeps silent about what he witnessed out of fear for his family's safety.

Miss Skade is an observant nurse who notices a man matching Carson Drew's description getting into a taxi. Her willingness to step forward and contradict the official narrative of the taxi drivers provides Nancy with a critical lead.

Samuel Greenman is a wanted criminal with a distinctive wrinkled or "crinkly" left ear. He is directly involved in the plot against Carson Drew and leaves a trail by purchasing a vehicle from a local used-car dealer.

Captain Rossland is a local police official who coordinates the official response to the disturbances at Twin Elms and the search for Carson Drew. Recognizing Nancy's unique abilities, he eventually allows her to question uncooperative witnesses that his officers cannot crack.

Tom Patrick is the police guard assigned by Captain Rossland to patrol the Twin Elms estate at night. He is stationed there to prevent further intrusions but struggles to find the source of the disturbances.

Anthony Barradale is a lawyer involved with the railroad dispute alongside Carson Drew. He communicates with the Drew household about the impending deadline for the settlement with the landowners, providing crucial information about the legal stakes.

Dr. Morrison is Miss Flora's local physician. He treats her for severe shock resulting from the hauntings and quickly dismisses her reports of an unshaven intruder as simple hallucinations.

Mr. Dodd is the local realtor responsible for Riverview Manor, the vacant property adjacent to Twin Elms. He inadvertently provides critical information regarding recent property sales in the area.

Albert Watson is a local man temporarily held as a trespasser on the estate grounds. He quickly proves he is merely an innocent neighbor taking a shortcut home.

Jim Archer is Helen Corning's secret fiancé. Helen keeps their engagement hidden from her family, providing a contrast between her conventional domestic plans and Nancy's independent lifestyle.