Madison Hunter (Knightley)

Madison Hunter, mostly referred to as Miss Hunter and Maddie is the childhood friend of John Watson. She is the half-sister of Irene Adler through their mothers, who is American. Madison’s father is British, meaning that Madison is half-British, half-American descent.

Instead of her being a love interest of Doctor Watson, Madison acts as love interest of Sherlock Holmes.

Irene Adler
Irene is Hunter’s half-sister, sharing the same mother but different fathers. Their mother is Isabelle Adler, who in return is the wife of Madison’s step-father, Joseph Adler. While Irene was raised by her mother, Madison was raised by her father. Irene seems to merge in with Madison’s brother and sister from the original novels. The two of them had different relationships, but Madison was enraged when she learned that James Moriarty killed her half-sister and wanted to make him pay for what he did.

John Watson
Watson and Hunter have been best friends since they were children and it isn’t all it's cracked up to be when they meet Sherlock Holmes for the first time. They do not feel any love towards one another and only feel a mutual friendship.

Sherlock Holmes
Instead of the original Doctor Watson acting as a love interest for Miss Hunter, she finds herself attracted to her half-sister’s love interest, Sherlock Holmes. Together, they have a child together, even Sherlock knew by reading the signs of Madison’s change in nature.

Role in A Game of Shadows
In A Game of Shadows, Madison learns she is pregnant with Holmes’ child; however she keeps it to herself in fear that she and the baby are harmed because James Moriarty is on the loose. Later on when Sherlock and Moriarty supposedly fall to their deaths, Madison is oblivious of her lover’s survival. However, Mycroft is told by his younger brother to look after Hunter and the baby while he is away before he can reunite with her.

Watson and Mary also cancel their honeymoon trip when Madison goes into labor eight months later to give birth to a baby boy she names after his supposedly late father, Sherlock Holmes and the baby has Watson’s first name as a middle-name.

Trivia

 * Unlike canon Hunter, Madison has a child with Sherlock and not John Watson, and has a son rather than a daughter. This is later adapted again in the 2010 version of the series where she has a son and names him Hunter, but has  the child with Watson after Mary's death.
 * Hunter is one of the few people who call Watson by his first name.
 * Irene Adler is revealed to be her half-sister.