Professor James Moriarty, the arch-enemy of Sherlock Holmes, is a mathematics professor turned the world's only 'consulting criminal'. His genius is equal to, if not perhaps greater than, Holmes himself.

Moriarty is a machiavellian criminal mastermind whom Holmes describes as the 'Napoleon of crime'. Conan Doyle borrowed this phrase from a Scotland Yard inspector who was referring to Adam Worth, a real-life criminal mastermind, who is one of the individuals upon whom the character of Moriarty was based.

The character was introduced primarily as a narrative device to enable Conan Doyle to kill Sherlock Holmes, and only featured in two of the Sherlock Holmes stories.

Moriarty is a genius, a philosopher and an abstract thinker, with a brain of the first order. As Holmes puts it:

β€œHe sits motionless, like a spider in the centre of its web, but that web has a thousand radiations, and he knows well every quiver of each of them.”

- Sherlock Holmes

Despite only appearing in two stories, Moriarty has been proven to be the most dangerous of all criminals that Holmes encounters. In the short story 'The Adventure of the Final Problem', during a fight with Holmes above the Reichenbach Falls, Moriarty falls to his death.

Irene Adler, professionally known as 'The Woman', is a brilliant dominatrix who has an apparently romantic attraction to Sherlock Holmes.

Whilst Mycroft – Holmes' elder brother (by seven years) – is telling Sherlock about Irene, he states that she is a dominatrix who gives out 'recreational scolding' to people. She has been in the middle of two political scandals in recent years, one of them involving a famous author, in which she had an affair with both sides.

She is brought to Sherlock's attention when he and John are summoned to Buckingham Palace by Mycroft and are asked to take on a case of national importance. Irene had taken compromising images of a young female member of the British Royal family during a dominatrix session.

Even though Irene does not want money or power for the pictures, Sherlock is tasked in getting them back. She is using them, and other information for her 'protection'.