Army surgeon invalided home from Afghanistan who becomes Sherlock Holmes's companion, lodger, and chronicler. Steady, decent, and possessed of considerable courage, he serves as both the practical counterweight to Holmes's eccentricity and the reader's point of entry into a world that would otherwise be entirely alien. Narrator of the overwhelming majority of the canonical stories.

The world's only consulting detective, operating from 221B Baker Street, London. Possessed of extraordinary powers of observation and deductive reasoning, he applies a rigorous scientific method to criminal investigation while remaining largely indifferent to conventional social expectations. Capable of brilliant warmth and profound coldness in equal measure, his partnership with Dr Watson is the central relationship of the canon.

Former Colonel of the First Bangalore Pioneers and big game hunter, described by Holmes as the second most dangerous man in London after Moriarty. Chief of staff to Moriarty's criminal organisation. Appears in The Adventure of the Empty House.

A former Professor of Mathematics turned criminal mastermind, described by Holmes as the Napoleon of Crime - the organising intelligence behind much of London's serious criminality. Appears directly in only a handful of stories but casts a shadow across the later canon.

Scotland Yard's most prominent inspector in the canon and Holmes's most frequent professional contact within the official police. Tenacious and not without competence, but habitually out of his depth on the cases that matter most, he represents the institutional approach to detection that Holmes consistently outpaces. His relationship with Holmes evolves from grudging tolerance to genuine respect across the canon.

An American opera singer and adventuress who appears in a single story, A Scandal in Bohemia, but earns a permanent place in the canon. Referred to thereafter simply as The Woman. Her prominence in adaptation far exceeds her actual canonical presence, but her impact on Holmes's characterisation is genuine and lasting.

Landlady of 221B Baker Street and the domestic anchor of Holmes's unconventional household. Tolerates her tenant's erratic hours, dangerous visitors, and destructive experiments with remarkable equanimity. More resilient and resourceful than her role suggests, she is a constant presence across the canon without ever being its focus.

Sherlock Holmes's elder brother, possessed of even greater powers of observation and deduction but entirely lacking his sibling's energy or inclination to act on them. A founding member of the Diogenes Club and a figure of considerable, deliberately vague importance within the British government - Holmes suggests at one point that Mycroft occasionally is the British government. Appears rarely but significantly.