Flavia is an eleven-year-old amateur sleuth living in the sprawling Buckshaw manor in 1950s England. A prodigy with a passion for chemistry, particularly the study of poisons, she finds solace in her private laboratory. She relies on her sharp intellect and acute observational skills to compensate for her father's emotional distance and the regular hostility of her older sisters. When a mysterious death occurs in her family's garden, she treats the investigation as an irresistible intellectual puzzle.

Colonel de Luce is Flavia's emotionally distant father and a former military man who served in World War II. He retreats into his extensive stamp collection to avoid the pain of mourning his late wife, leaving him detached from his three daughters. Though he projects the image of a fearless British gentleman, unexpected events from his boarding school past deeply unsettle him.

Often referred to as Dogger, Arthur Wellesley is the de Luce family's gardener and general handyman. He served as a medical orderly during the war and survived time as a prisoner of war, leaving him with severe trauma that manifests as terrifying hallucinations and physical tremors. He acts as the sole nurturing adult figure in Flavia's life.

Horace Bonepenny is a former schoolmate of Colonel de Luce. During their time at the Greyminster boarding school, he was a pale, bony student who shared a talent for magic tricks with the Colonel before growing cold and manipulative. He arrives at Buckshaw years later to demand money, setting off a deadly chain of events.

Robert Stanley is a former student from Colonel de Luce's youth at the Greyminster school. Remembered as an older boy with decidedly unsavory characteristics, he exerted a corrupting influence over his peers, particularly Horace Bonepenny. His past actions cast a long shadow over the current events in Bishop's Lacey.

Inspector Hewitt is the lead police detective handling the investigation in Bishop's Lacey. He approaches his work meticulously and operates by the book, but he also possesses enough creative thinking to recognize Flavia's unusual intelligence, eventually treating her observations with genuine respect.

Ophelia is Flavia's seventeen-year-old sister. She plays the piano beautifully and spends much of her time reading movie magazines, dreaming about local boys, or obsessing over her appearance. She frequently locks Flavia in closets or belittles her, using her clearer memories of their deceased mother as a weapon in their sisterly disputes.

Daphne is Flavia's thirteen-year-old sister. She is constantly absorbed in reading classic literature and murder mysteries, using books as her own method of escaping the gloomy atmosphere of Buckshaw. She typically sides with Ophelia during household arguments and largely ignores Flavia's attempts at communication.

Harriet is Flavia's deceased mother who died in a mountaineering accident when Flavia was an infant. Her memory dominates the estate, where her possessions, including her Rolls Royce and her meticulously preserved dressing room, remain untouched. Her absence leaves a profound emotional void that shapes the behavior of her surviving family members.

Mrs. Mullet is the de Luce family cook who prepares heavy, often unappreciated meals, including her notorious custard pie. She provides comic relief in the household and serves as Flavia's primary source of village gossip.

Mr. Twining was a housemaster at the Greyminster school during Colonel de Luce's youth. A kindly educator who organized a Philatelic Society and a Magic Circle for his students, his career ended tragically when he apparently plummeted from the school's clock tower.

Miss Mountjoy is the retired librarian of Bishop's Lacey, known for her fiery temper. She harbors deep bitterness over the tragic death of her uncle, believing that wealthy students escaped justice for their role in his demise.

Dr. Kissing is the former headmaster of Greyminster. Once the proud owner of an exceedingly rare postage stamp, he now resides in a private hospital. He retains a sharp mind and recognizes Flavia's intelligence when she seeks him out for answers.

Ned is a young man who works at the Thirteen Drakes inn in the nearby village. He harbors a quiet affection for Ophelia de Luce and sometimes clashes with his coworker over workplace issues.

Mary is the innkeeper's daughter who works at the Thirteen Drakes. She initially views Flavia with suspicion due to class differences, assuming the wealthy girl looks down on her, but warms up when she realizes they share a similarly dark sense of humor.

Tully is the boisterous proprietor of the Thirteen Drakes Inn in Bishop's Lacey. He manages the establishment loudly and occasionally interacts with the local police regarding his guests.

Miss Cool is the proprietor of the local confectionary shop in Bishop's Lacey. She provides Flavia with both acid drops and useful directions during the young detective's village inquiries.

Dr. Darby is the local doctor and coroner for Bishop's Lacey. He works alongside Inspector Hewitt during criminal investigations, providing medical estimations and examining evidence at the scene.

Maximilian is a retired musician and neighbor to the de Luce family. He enjoys chatting about his past concert tours and inadvertently provides Flavia with geographical facts crucial to her investigation.

Alf is the husband of the de Luce family cook. He served in the military alongside Arthur Wellesley during the war, establishing a historical connection to the Buckshaw household.