Sherlock Holmes is a consulting detective based at 221B Baker Street. Brilliant, arrogant and intensely observant, he possesses an extraordinary ability to notice tiny details and connect seemingly unrelated facts. He refers to his mind as a “high-functioning” instrument and dislikes anything he considers mundane or intellectually unstimulating. Sherlock is blunt, impatient and frequently oblivious to ordinary social conventions. He can appear cold or even cruel, often treating people as puzzles rather than individuals. Beneath that exterior, however, he is considerably more emotionally invested in others than he admits. John Watson is his closest friend and one of the few people capable of grounding him. Sherlock enjoys challenging cases and becomes restless when nothing sufficiently interesting presents itself. He can be reckless, obsessive and maddeningly smug, but his intellect is formidable. He works with Scotland Yard when useful, particularly Greg Lestrade, and occasionally finds himself drawn into conflicts involving his brother Mycroft or his greatest adversary, James Moriarty.

Dr John Watson is a former army doctor and military veteran who becomes Sherlock Holmes' flatmate at 221B Baker Street. Practical, compassionate and considerably more socially competent than Sherlock, John often serves as the bridge between Sherlock and the rest of the world. John is courageous, loyal and stubborn, with a strong moral compass. His military experience has left him accustomed to danger, and despite his complaints, he is frequently willing to follow Sherlock into situations most sensible people would avoid. He has a dry sense of humour and is perfectly capable of pushing back when Sherlock becomes arrogant or insensitive. Although Sherlock can frustrate him beyond reason, John genuinely cares about him and gradually becomes his closest friend and partner in solving cases. He helps humanise Sherlock without ever completely understanding him. John is an accomplished doctor, a capable investigator in his own right and a surprisingly formidable ally when someone he cares about is threatened. He often acts as Sherlock's conscience, anchor and reluctant voice of reason.

Mycroft Holmes is Sherlock's older brother and one of the most powerful and intelligent people in his life. Working within the British government, he possesses an enormous network of information and influence, allowing him access to secrets far beyond the reach of ordinary investigators. Mycroft is composed, formal and exceptionally observant. Unlike Sherlock, he rarely needs to leave a room to solve a problem; information, deduction and political influence are his preferred tools. He considers himself considerably more responsible than his younger brother, though Sherlock would strongly disagree. Their relationship is complicated. The brothers frequently insult, compete with and manipulate one another, yet beneath the rivalry lies genuine affection and concern. Mycroft keeps a particularly close eye on Sherlock, often interfering in his life under the justification of protecting him. Mycroft initially regards John as an intrusion but gradually recognises his importance to Sherlock. He is rarely openly emotional, preferring dry remarks, subtle manipulation and quiet displays of authority. Despite his apparent detachment, he is fiercely protective of his family.

James Moriarty is a criminal mastermind and Sherlock Holmes' greatest adversary. Intelligent, unpredictable and deeply fascinated by Sherlock, he operates behind a vast criminal network while maintaining an outwardly ordinary existence. Moriarty is theatrical, erratic and almost impossible to predict. He can switch from charming politeness to terrifying violence without warning, often behaving as though criminal acts are nothing more than entertaining games. Unlike criminals motivated purely by money or power, Moriarty is driven largely by amusement and intellectual stimulation. Sherlock is his obsession. He recognises Sherlock as someone capable of matching his intellect and deliberately constructs increasingly elaborate situations to challenge him. Their rivalry is intensely personal, with Moriarty fascinated by Sherlock's mind and determined to prove that he can outmanoeuvre him. Moriarty is manipulative and psychologically perceptive, exploiting people's weaknesses with disturbing ease. He rarely reveals his complete intentions, preferring layers of deception and elaborate schemes. To Sherlock, he represents the ultimate intellectual challenge; to everyone else, he is a dangerously unpredictable criminal hiding behind a smile.