Sherlock Holmes, played by Benedict Cumberbatch
The modern-day Sherlock Holmes is "cold and clever". (However, astronomy is not useful in solving crimes, Sherlock is unaware that the sun revolved around the Earth). An outcast even as a child, Sherlock spends his time observing everything, analyzing it. He's driven by a desire to prove himself cleverer than everyone: the police and the criminals alike.
Holmes is well aware of his own weaknesses, describing himself as a "high-functioning psychopath." However, he cares nothing for the opinions of others, including his brother Mycroft.
Sherlock is skilled at computer hacking and information retrieval, and maintains his own web site. Sherlock is also a skilled hand-to-hand fighter.
As the world's only consulting detective, Sherlock's primary skill is his ability to analyze small clues and compile them into a complete profile of a crime scene. Although he doesn't abide by normal human standards of behavior, he's capable of applying his observations to analyze it. While Sherlock may not conceive of the idea of marriage or adultery, he can look at a woman's wedding ring, measure how it's been cleaned inside and outside of the band, and determine that the woman was having multiple affairs. (episode 1 season 1)

John Hamish Watson also called, Dr. John Watson, is a 37 old man. He is often complete different from Sherlock in both appearance and personality. Unlike Sherlock, John is short with blond hair. He is kind, caring, and 'human' compared to Sherlock's cold, calculating ways. He has served in Afghanistan as an army doctor, but he has also in some way been a soldier, since he has a limp and supports himself to stick. But that only lasts the first episodes, since he suddenly leaves the stick in a restaurant running after a taxi with Sherlock and at the end it's revealed that he wasn't shot in the leg but the shoulder, and that he's perfectly fine.
Even after several warnings from everyone about Sherlock, much from Sergeant Sally Donovan, he still stays with Sherlock, actually liking the other. Liking the excitement from finally being back in the war again.
Watson has his own blog, where he writes about his adventures with Sherlock, which they often talk about in the series.
Mycroft; You're not haunted by the war, Dr Watson. You miss it. Greg Lestrade;
Works for Scotland Yard and directly with Sherlock. He's a police officer and he trusts Sherlock a lot. He believes that Sherlock is innocent and he hopes that Sherlock is a good man, though in season two it's proven otherwise to Lestrade's surprise.
And then there's Sergeant Sally Donovan, who dislikes Sherlock a lot, and Anderson who would agree with her fully. Sherlock also figured out that they had an affair in episode one to both their embarrassments. James 'Jim' Moriarty
Moriarty is the only "consulting criminal" in the world, a counterpoint to Sherlock's similarly unrivalled "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.
Meanwhile, Moriarty proceeds to break into the case where the Crown Jewels are kept, while simultaneously opening the vault at the Bank of England and unlocking all the cells at Pentonville Prison via his mobile phone. Before smashing the Crown Jewels' case, he writes the words "Get Sherlock" on the outside, to be seen by the security cameras. He then allows himself to be caught wearing the jewels and sitting on the throne. This show how much power he has.

Mrs Hudson 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. She takes good care of him, and in the start when she met John, she thought John was Sherlock's romantic partner until proven wrong.
Sherlock has a great deal of concern for her. In "A Scandal in Belgravia", Sherlock pushed an American who beat her out of a second-story window to "restore balance to the universe"; apparently, this was done so many times that "[Sherlock] lost count."
EPISODE 1 SEASON 1:
A series of suicides leaves the police on a bare bottom. The suicides have no connection at all other than being found at deserted locations. The police officer Lestrade goes to Sherlock, who has just met John who accompanies him to a crime scene. It is a lady dressed in pink from top to toe, a nails are pink and she has left a message on the floor. From there Sherlock picks up that she was on a one day trip, and that she had a bag which is gone, her phone too. Sherlock also figures that it's a serial killer, and that he has finally made a mistake, so now Sherlock can find him.
Sherlock Holmes: They're killings, serial killings! We've got a serial killer on our hands. I love those, there's always something to look forward to.
Sherlock finds her bag not far away from the crime scene, and he fuzzes about not having found her phone yet. Then after a lot of thinking he realizes that the message the woman had carved into the floor, saying Rache..., is actually Rachel, and that is the password for a website which can locate your phone and he does. The website though tells him that the phone is in the very same room, and someone had just called a taxi. Sherlock runs downstairs, having figured it out enters the taxi. The taxi driver who's behind all the killings has been doing this, that he took the people to deserted locations and gave them a choice between to bottles of pills. He would be taken one and they would be taking another. One is deadly, the other isn't. He had threatened the people with a fake gun, though they didn't know, and they had chosen one and taken a pill that eventually killed them. He does the same thing to Sherlock, though Sherlock is quick to notice the gun is fake, and when he takes the pill, nearly taking it, the old man is shot. Watson stood in a house beside the empty building and shot the man. Though no one caught up on the fact that it was him other than Sherlock. They both keep it a secret.
Sherlock Holmes: Shut up.
DI Lestrade: I didn't say anything.
Sherlock Holmes: You were thinking. It's annoying.

Dr. John Watson: What are you doing?
Sherlock Holmes: Nicotine patch. Helps me think. It's impossible to sustain a smoking habit in London these days.
Dr. John Watson: That's good news for breathing.
Sherlock Holmes: Ah, breathing. Breathing's boring.
Dr. John Watson: Is that three patches?
Sherlock Holmes: It's a three-patch problem.

Sherlock Holmes: They're killings, serial killings! We've got a serial killer on our hands. I love those, there's always something to look forward to.
Setting:
The series is primarily filmed in Cardiff, although the production also uses a variety of other locations. North Gower Street in London was used for exterior shots of Holmes and Watson's 221B Baker Street residence.

Actually, I don't believe that Holmes and Watson are "good friends". I think their relationship is something more of a partnership, something that runs quite deep. Holmes trusted Watson far more than anyone else. Holmes trusts Watson with information he doesn't even relay to Lestrade, who is an official investigator and who has a history of working on cases with Holmes. Watson is not only a confidant, but also someone who never doubts Holmes.