A brilliant, 15-year-old orphaned girl with a fierce intellect and feminist sensibility. She lives under the guardianship of an emotionally volatile aunt but finds a sense of purpose when she meets Sherlock Holmes. She studies theology and chemistry at Oxford University while training in deduction, forensic science, and disguise.

The legendary detective, now in his mid-fifties, who has retired to the countryside to study apiculture. Solitary and physically declining until he meets Russell, his keen intellect is revitalized by her presence. He transitions from a detached reasoner to an active, emotionally invested mentor as he trains his new apprentice.

Holmes's former investigative partner and biographer. He is steadfast and loyal, possessing a large heart that contrasts with the intense intellect of Holmes and Russell. He represents an uncomplicated, foundational goodness that grounds the more eccentric characters.

The highly competent and imperturbable housekeeper for Sherlock Holmes. She provides vital domestic stability and quiet strength, functioning as a grounding force in the eccentric household and an essential maternal presence for the young apprentice.

A brilliant mathematics tutor at Oxford University who introduces Russell to complex mathematical concepts, including base-eight number theory. She possesses a superb academic mind and serves as a mentor figure during Russell's university studies.

Sherlock Holmes's older brother, a powerful figure operating in the highest echelons of the British government. He handles matters of intelligence and political maneuvering, providing vital resources and strategic support from behind the scenes.

The phlegmatic, loyal manager of Russell's family farm. He knew her mother and initially treats Russell formally, but comes to deeply respect her when she insists on engaging in the physical labor of the harvest.

An energetic and vivacious student at Oxford who draws the solitary Russell into university social life. Often wrapped in gaudy silks, Ronnie introduces her friend to amateur dramatics and encourages her penchant for bold pranks.

A wealthy, elegant neighbor of Holmes who brings the duo their first official case. She is deeply concerned about her husband's mysterious, recurring illnesses and seeks unconventional help to uncover the cause.

A government advisor with access to sensitive intelligence. He suffers from mysterious bouts of illness on clear nights, prompting his wife to seek investigative help to ensure he is not compromised.

The manservant employed by Richard and Mrs. Barker at their country estate. He tends to the household and provides direct care to Mr. Barker during his recurring bouts of illness.

An American senator visiting Wales with his family. He becomes desperate for professional help when his young daughter goes missing during their countryside vacation.

The distraught mother of Jessica and wife of an American senator. She seeks answers and justice after her family is incapacitated and her young daughter vanishes from their campsite.

The intelligent, six-year-old daughter of an American senator. Her disappearance prompts an intensive undercover investigation, providing Russell an opportunity to exercise her emotional intuition alongside deductive reasoning.

A Scotland Yard inspector who frequently intersects with Holmes's investigations. He coordinates official police efforts, secures evidence, and provides law enforcement backing when cases escalate.

The head of the local police in Wales, who assists Holmes and Russell by providing official case files regarding the high-profile Simpson kidnapping.

The owner of the Monk's Tun inn in Sussex. The burglary of her business leads to Mary Russell taking on her very first independent local investigation.

A new employee at Tillie Whiteneck's local inn. His background and residence become a point of interest during an investigation into stolen goods.

A local constable in Sussex who insists on waiting for Sherlock Holmes to arrive before allowing Mary Russell access to a local crime scene, much to her frustration.

The porter at Mary Russell's university lodgings. He handles the mail and manages the front entrance to the building.

A covert operative working for Mycroft Holmes who captains the vessel that transports Holmes and Russell out of immediate danger and across the sea.

An English intelligence agent operating in disguise as a local guide in the Middle East. He assists Holmes and Russell during their period of exile.

An English intelligence agent utilizing a disguised persona to operate securely in Palestine. He provides logistical support and guidance to Holmes and Russell.