- Sherlock Holmes (Benedict Cumberbatch) is a tall, thin man with dark, curly hair. As with the original character, Sherlock is able to deduce (or rather, abduce) information from the smallest details. He describes himself as “A consulting detective, the only one in the world”, consulted by Scotland Yard when they are out of their depth with cases (usually homicides). Sherlock is often insensitive of how this affects the officers and usually ridicules their incompetence. However, he is frequently mocked by them for being eccentric, especially by forensics member Anderson and Sergeant Sally Donovan.
Sherlock seems to care more about the crimes than the victims involved, like the Holmes of the original canon. He is not very sociable, nor does he suggest that he has friends. Sherlock is rude and inconsiderate towards others and to an extent, his flatmate and colleague John, often leaving him behind and asking him to do menial, domestic tasks such as shopping. However, Sherlock is shown to care deeply for John as the series progresses, showing uncharacteristic emotion when his friend's life is in jeopardy or when John comes to his aid.
Sherlock also has considerable fighting ability, and fights off a man armed with a sword with his bare hands in "The Blind Banker". He is also adept with a gun and able to chase a cab on foot across London.
He considers only three people as his friends: John Watson, Mrs Hudson, and DI Greg Lestrade. The only other person he seems to trust fully is Molly Hooper, as seen in The Reichenbach Fall.
Romantically, Sherlock appears largely asexual (though Cumberbatch has mentioned this is not the case), self-described as "married to his work"; he is oblivious or disdainful of 'love'. Sherlock ignores the romantic interest that forensic pathologist Molly Hooper has in him, but sometimes uses this to his advantage if it assists him in a crime investigation. In season 2, after the introduction of Irene Adler, Sherlock appears to have some interest in Irene and comes to her rescue at the end of the episode. She frequently flirts with and texts Sherlock in A Scandal in Belgravia, but he seldom responds.
He knows London's streets and alleys extremely well, and has strange connections, including a graffiti artist as well as the entire network of the homeless, whom he uses as his "eyes and ears."
Sherlock is a smoker who finds the habit impractical in present-day urban London and therefore mostly makes do with nicotine patches. He may apply as many as three patches on himself at the same time; he implies that the nicotine helps him to think. For the same professed reason, he plays the violin and even composes music from time to time. It is also implied that he has a history of illegal drug use. Sherlock has violent mood swings during periods of extreme ennui when there are no cases that interest him. He is once seen curled up on the sofa and firing a gun at the wall in his flat. In "A Study in Pink", he claims to be a "high-functioning sociopath".
- John Watson (Martin Freeman) is Sherlock's flatmate. He is often a foil to Sherlock in both appearance and personality. Unlike Sherlock, John is short, with blond hair. As well, he is kind, caring, and 'human' compared to Sherlock's cold, calculating ways. He gets on better with the police and takes care of practical matters at their flat, apparently doing most of the shopping. He also gets a locum job at nearby general practice to help pay the bills, and starts dating Sarah, another doctor he meets working there.
At the start of the series John was recently invalided home from Afghanistan after serving as an "army doctor" (It is unclear whether John served separately as a doctor then a soldier, or as a medic). Prior to his military service, John read medicine at King's College, London as seen on his CV. He is in possession of a pistol, described as a "British Browning L9A1" in the show, but is actually a Sig Sauer P226. John is highly proficient with the weapon, described as a "crack shot" by Sherlock.
Before meeting Sherlock, and after returning to London from Afghanistan, John had several medical issues: a psychosomatic limp in his right leg, and bullet wound in his left shoulder (a reference to the canon), using a cane on his right side. As well, his left hand had an "intermittent tremor". His therapist noted John had "trust issues" and diagnosed him with PTSD, the latter of which Mycroft (Sherlock's brother) later subverted, saying John is not "haunted by the war" but rather misses it for the excitement. Following a chase around London with Sherlock, John's limp disappears (and so, presumably, does the tremor).
John is also shown to be loyal and courageous, with a strong sense of personal integrity. His "strong moral principles" are likely a result of his medical training and army service. He refuses Mycroft's offer to pay him to spy on Sherlock, despite only having just met Sherlock at this time (and not knowing that Mycroft was his brother). He is willing to occasionally use deadly force, but he is only ever seen to do so when someone else's life is in immediate danger. He shows no visible signs of regretting his actions afterwards.
In the second series John is referred to as Sherlock's P.A. and is not shown working any other job. He is alluded to have had a number of short-lived relationships with women - one of whom is shown ending their relationship due to John's commitments to Sherlock, saying, sarcastically, that "[Sherlock] is a very lucky man."
The only member of John's family that has been alluded to thus far is his sister, Harriet (aka Harry), with whom he has strained relationship as a result of her alcoholism and recent divorce. During their first case together Sherlock deduced that John had refused to accept help from Harry after being invalided out of the army, even though the fact that she gave John her old phone was an indication that she wanted to stay in touch with her brother.
Watson's middle name being Hamish was originally proposed by Dorothy L. Sayers to explain discrepancies in Watson's name in the canonical works of Conan Doyle.
- DI Greg Lestrade (Rupert Graves) works for Scotland Yard and directly with Sherlock. He has a reluctant respect for Sherlock and none of the animosity his team seems to have. However, he is often frustrated by Sherlock's cryptic deductions and withholding of evidence. Lestrade tries to get his team to cooperate with Sherlock whenever possible, even when the man is being insulting.
- Jim MoriartyThe primary antagonist of the series, James 'Jim' Moriarty (Andrew Scott) is the only "consulting criminal" in the world, a counterpoint to Sherlock's similarly unrivaled "consulting detective". He is also responsible for the criminals and crimes in all three episodes of the first series, acting as a sponsor, an informant, or a mastermind. He is shown to have an interest in Sherlock that borders on obsession, though he does not hesitate trying to kill him when he loses interest. He is a highly capable, entirely psychopathic, and extremely powerful criminal. Moriarty's primary motivation appears however to be the avoidance of boredom rather than a thirst for money or power, which is actually a very common 'problem' for psychopaths.
He is also an excellent actor, frequently shown to have convinced others that he is someone else - indeed his first appearance occurs as a man who Molly is dating, and Sherlock does not realise his identity at the time. In "The Great Game," Moriarty forces Sherlock to solve mysteries within a time limit, taking hostages to ensure that Sherlock is sufficiently motivated.
In the second series he continues in his role as consulting criminal, giving Irene Adler advice on how to manipulate the "The Holmes Boys," having given them both nicknames, it is also mentioned that he asked for nothing in return, becoming involved just to cause trouble, suggesting his obsessions are deepening. It is also alluded to that he might have an independent rivalry with Mycroft. He is seen briefly at the end of "The Hounds of Baskerville" apparently having been captured and in the process of being released by Mycroft.
Moriarty features extensively in "The Reichenbach Fall", simultaneously breaking into the Tower of London, HMP Pentonville and the Bank of England, being tried but acquitted after intimidating the jury, and then setting about to destroy Sherlock's reputation and ultimately force him into suicide. When Sherlock realizes there is a way out as long as Moriarty is alive, Moriarty shoots himself in the head in order to force Sherlock into committing suicide.
- Mrs. Hudson (Una Stubbs) is the landlady of 221B Baker Street. Sherlock won his way into her good graces after ensuring that her husband was executed for an unknown crime in Florida. Despite repeated declarations that she is not their housekeeper, she fusses over Sherlock and he takes it for granted that she will provide dinner for him.
- Mycroft Holmes (played by executive producer, co-creator, and writer Mark Gatiss) is first introduced when he kidnaps John and offers to pay him to spy on Sherlock (out of brotherly concern of course). He is Sherlock's older brother and engages in sibling rivalry with Sherlock. Mycroft is frequently mocked by Sherlock for "putting on weight". He occupies a "minor position in the British government"; however, as with many Holmes-based works, it is heavily hinted that he has a much bigger role than he claims. He is driven around in a private car with his personal assistant who goes by the name of "Anthea". Mycroft, like his literary counterpart, is also very skilled at deduction, even correcting Sherlock on occasion, as well as the lack of enthusiasm towards "legwork".
- Molly Hooper (Louise Brealey) is a pathology lab assistant working in the morgue at St Bartholomew's Hospital with an apparent crush on Sherlock. Due to her work position and crush, Sherlock frequently exploits her to let him examine victims' bodies. She was in a relationship with an IT employee named Jim, who was later revealed to be Jim Moriarty.
Molly was originally intended to be a one-off character to introduce Sherlock, but Brealey impressed Moffat and Gatiss and they "couldn't resist bringing her back"
- Irene Adler (Lara Pulver) features in "A Scandal in Belgravia" as a bisexual dominatrix who repeatedly outwits Sherlock before being finally defeated herself. She sends Sherlock a series of flirtatious texts, repeatedly requesting to "have dinner" with him.