Abigail is a pragmatic young Englishwoman who abandons her strict upper-middle-class upbringing and an unfulfilling paleontology dig to seek adventure in America. Possessing a keen eye for mundane details and an unyielding curiosity, she balances Jackaby's eccentricity with practical deduction. She values independence and actively rebels against the societal limitations placed upon women of her era.

R. F. Jackaby is an eccentric paranormal investigator operating out of a chaotic house at 926 Augur Lane. Gifted with the unique ability of second sight, he sees the magical elements of the world that remain hidden to ordinary people. He displays profound compassion toward misunderstood creatures but struggles significantly with human emotional cues and societal norms.

Charlie is a young, hardworking police officer on the New Fiddleham force. He is deeply loyal to his superiors but displays a rare open-mindedness toward Jackaby's unconventional investigative methods. He carries a heavy burden of responsibility, striving to protect innocent citizens while hiding a profound personal secret regarding his true nature.

Swift is a pompous, highly ornamented police commissioner who wields significant political power in New Fiddleham. He wears heavy metal braces on his legs and an eye-catching red hat. He uses his high-ranking position to pressure Marlowe for quick results, driving the public narrative around the ongoing murder investigation.

Jenny is a ghost residing in Jackaby's eclectic home on Augur Lane. Before her death, she was a scientifically-minded woman who studied alongside her fiancé. She maintains a fastidious, caring presence in the household and provides Abigail with much-needed female companionship in an otherwise chaotic, male-dominated environment.

Marlowe is the head of the local police investigation and a highly practical law enforcement officer. He relies on conventional evidence, making him skeptical of Jackaby's supernatural theories. Despite his gruff exterior and impatience with eccentric behavior, he possesses a genuine desire to uphold the law and protect the citizens of New Fiddleham.

Hatun is an unhoused woman who possesses a form of second sight, allowing her to perceive supernatural entities wandering the streets. She wears a unique knitted shawl that renders her virtually unnoticeable to anyone possessing a permanent home. She acts as a quiet observer of the city's hidden magical occurrences, trying in her own way to protect the town.

Douglas is Jackaby's former investigative assistant who resides permanently in an indoor pond. Due to a magical mishap during a past case, he lives entirely in the physical form of a duck. He serves as a humorous fixture in the household, surprisingly content in his current avian state despite Jackaby's insistence on finding a cure.

Mr. Henderson is a nervous resident living in the same apartment building as the first murder victim. He suffers from terrible anguish because he hears a constant, agonizing woman's scream that no one else in the building can detect.

Mona is a fiery, highly protective Irish woman living in the apartment above Mr. Henderson. She serves as the devoted caretaker for her elderly relative and reacts aggressively to strangers interfering with her family's affairs.

Mrs. Morrigan is an elderly Irish woman who suffers from chronic, agonizing silent seizures. She is secretly a banshee, an ancient household spirit whose silent wails serve as a warning of tragedy for those residing nearby.

Arthur Bragg is a local journalist who helped Commissioner Swift manage his political publicity. He uncovers a dangerous mystery, and his brutal murder in his apartment kicks off the central investigation of the novel.