Original Background Story Edit
Irene Adler is a character in the NPLH universe (and the original stories by Sir Arthur Conan Doyle).
According to "A Scandal in Bohemia," Adler was born in New Jersey and followed a career in opera as a contralto, performing at La Scala in Milan, Italy, and a term as prima donna in the Imperial Opera of Warsaw, Poland. She became lover to the King of Bohemia, but later held him to blackmail with a compromising photograph of the two.
Sherlock Holmes was brought in to get it back, but despite his best efforts, is unable to do so and Adler flees the country with her new husband, Godfrey Norton. But not before leaving the photograph and a letter promising to never speak of the affair again.
As a reward for his efforts, the King allows Holmes to keep the photograph. The item becomes a prized possession of his and a memento to the first woman to have ever bested him at a game of wits.
NPLH Universe Continuation Edit
Although the original story stops there, in NPLH the two do end up meeting again and becoming allies. After faking his death at the Reichenbach falls in 1891, Holmes takes up work as an actor on a cruise liner in Sweden and finds Adler working there. She claims to have tired of her husband and so chose to return to work. They strike up an alliance to look out for another and then continue to travel across Europe whilst evading Moriarty's men.
Irene Adler was originally played by Lucy Maslen, she actually filmed pieces for the first episode "Moving Forward", but these were cut from the original and she was later recast with the younger Swedish-American actress, Karin Rydle.
Adler first appears in some flashback moments of the 3rd episode of the 1st series "The Dubious Spiritualist".
In the flashback, she gives Holmes her necklace, another memento to remember her by, which is used by The Spiritualist to try and obtain a psychic reading for Holmes. When The Spiritualist is actually able to describe Adler to Holmes, he realizes he is not the con-artist he first thought he was.
Her final appearance was in the short Storm In a Tea Cup (Feb 2012).
Although the character is considered retired from the series, Holmes still keeps her photograph on the mantelpiece, a piece of information that Madeline was able to dig into to discover the truth about his past.
Alder is also a significant character in the Holmes In Time For Christmas Novel, with almost half of the book narrated from her perspective.