Holmes is an eccentric, brilliant man who created his own profession as the world's only unofficial consulting detective. He possesses immense knowledge of chemistry, crime, and forensic details, utilizing rigorous deductive logic to solve mysteries. When lacking complex problems to occupy his highly active mind, he relies on artificial stimulants to stave off boredom.

Watson is a former British Army surgeon who shares quarters at 221B Baker Street after being invalided from service in Afghanistan. He serves as Holmes's loyal friend and chronicler, documenting their investigative adventures. He maintains a more conventional, emotional outlook than his flatmate and quickly develops an affection for their new client.

Mary is a young, blonde governess who lives with her employer in London. Her father vanished ten years prior, and she has recently received instructions to meet an unknown person promising to right a past wrong. She maintains remarkable self-control and determination, displaying little concern for the prospect of sudden wealth.

Small is a middle-aged British man who previously lost his leg to a crocodile in India. He harbors an intense obsession with the Agra treasure and feels entirely entitled to the wealth he helped steal during the Indian Rebellion. His desire for revenge and wealth drives his relentless pursuit across the globe.

Thaddeus is the small, nervous son of the late Major John Sholto. Unlike the rest of his family, he does not harbor an obsessive greed for the Agra treasure and believes Mary Morstan deserves her fair share. He is highly anxious about his health, frequently complaining of heart issues and asking for medical reassurance.

Major Sholto is a retired British officer who served at a penal colony in the Andaman Islands alongside Captain Morstan. He returned to England immensely wealthy but lived in extreme paranoia, constantly employing guards and living in fear of men with wooden legs.

Bartholomew is Thaddeus's twin brother, residing at the family estate. He inherits his father's obsessive greed regarding the hidden Agra treasure, spending six years thoroughly searching and tearing apart the family house to locate the hidden wealth for himself.

Tonga is an Andamanese islander who accompanies Jonathan Small in London. Viewed through the highly prejudiced lens of Victorian society, he is treated as a dangerous and fearsome outsider who aids Small in his obsessive quest for the treasure.

Captain Morstan was a British guard at a penal colony in the Andaman Islands. He vanished without a trace ten years ago while visiting London, leaving his daughter to search for answers.

Athelney Jones is a police detective who has previously crossed paths with Sherlock Holmes. He favors quick, conventional conclusions and openly doubts Holmes's unorthodox methods, frequently rushing to arrest the most obvious suspect without analyzing deeper evidence.

Mrs. Forrester is Mary Morstan's employer in London. She treats Mary with great kindness, acting more as a supportive friend than a strict boss, and is familiar with Holmes's investigative skills.

Wiggins is a scruffy, streetwise boy who leads the Baker Street Irregulars, a group of working-class youths. He and his companions assist Holmes by gathering information across London's streets, acting unnoticed by the upper classes.

Mordecai Smith operates a boat rental service along the Thames River. His fast steam launch, the Aurora, becomes a critical piece of the puzzle as Holmes attempts to track movements on the water.

Toby is a tracking dog with an exceptional sense of smell. Holmes and Watson enlist his help to follow the strong scent of creosote left behind at the crime scene, leading them through the streets of London.

Mahomet Singh is a Punjabi soldier who, along with two others and Jonathan Small, originally conspired in India to steal the vast fortune from a courier.

Abdullah Khan is one of the original four men who plotted to steal the Agra treasure from a courier during the Indian Rebellion.

Dost Akbar is a co-conspirator involved in the original theft of the Agra treasure, forming the four-man pact that gives the story its central symbol.

Achmet is a courier sent by the Rajah of Agra to transport a massive fortune to safety. He falls victim to the plot hatched by Small and his accomplices.