William Sherlock Scott Holmes describes himself as "a consulting detective, the only one in the world," helping out Scotland Yard when they are out of their depth with cases (usually homicides). He appears as a tall, thin man with dark, curly hair. Like the original character, Sherlock is able to deduce or abduce information from the small details.

Sherlock has a very unusual personality, and describes himself in the first episode "A Study in Pink," as a "high-functioning sociopath," a term he insists on in subsequent episodes. (Others call him a psychopath and in "The Hounds of Baskerville," John tells Greg Lestrade that Sherlock has Asperger Syndrome.) This manifests itself in very poor people skills, and extremely rude manners to everyone he deals with. He is however skilled at coercing people to help him, by mostly playing on their weaknesses.

Sherlock seems to care more about the crimes than about the victims involved. He is rude and inconsiderate towards others, including 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. He even admits that saying Mary deserves John as a husband is the highest compliment he can give. He considers only three people as his friends: John Watson, Mrs Hudson, and DI Greg Lestrade, whose first name he repeatedly gets wrong. 

Sherlock appears largely asexual and describes himself as "married to his work". He seems oblivious to the romantic attraction that forensic pathologist Molly Hooper has for him, but sometimes uses this to his advantage if it assists him in a case. After the introduction of Irene Adler, Sherlock seems to have some interest in Irene and comes to her rescue. She frequently flirts with and texts Sherlock in "A Scandal in Belgravia," but he seldom responds. In the following series, Sherlock briefly has a mental image of her in his Mind Palace when thinking of who would know John's middle name, but quickly retorts "Get out of my head, I'm busy." 

Sherlock has considerable fighting ability, fighting off a man armed with a sword using his bare hands in "The Blind Banker." In "A Scandal in Belgravia," he disarms and incapacitates a CIA agent on two separate occasions; however, he is easily bested by John, who has military experience.

He knows London's streets and alleys extremely well; in "A Study in Pink", he quickly figures out what route a cab would take and plans a route to beat it to its destination. In "The Empty Hearse", Sherlock works out which paths to take to shave time off the countdown to save John. He has strange connections, including a graffiti artist, as well as the entire network of the homeless (whom he uses as his "eyes and ears"). In solving cases, Sherlock uses a technique known as the Method of Loci, which he calls his "mind palace," which enables him to efficiently store and retrieve information and make deductions. Initially the mind palace appears as a series of assorted facts that appear on the screen when a scene is showed from Sherlock's point of view. 

Sherlock has violent mood swings when there are no cases that interest him. He is once seen stretched out on his chair and firing a gun at a smiley face spray-painted on the wall in his flat. He finds smoking impractical in present-day urban London and, therefore, mostly makes do with nicotine patches. He may apply three patches at the same time when it is a "three patch problem", implying the nicotine helps him to think. For the same reason, he plays the violin and even composes music from time to time. Sherlock also has a history of recreational drug use.

Trying to avoid getting his face taken in pictures, Sherlock disguises it with a deerstalker. This becomes his trademark, much to his annoyance as he greatly dislikes the hat. However, he seems to have accepted it since he puts one on before greeting the press about his miraculous return in "The Empty Hearse".