Circa 1887, Victorian England - Mycroft Holmes defining trait is not his brilliance (which overshadows Sherlock's) but his laziness. Conan-Doyle has suitably placed him as an government official (auditing dept.) at Whitehall, and unsurprisingly - he is fat (Heavily built and massive, there was a suggestion of uncouth physical inertia in the figure). Mycroft resides at Pall Mall - less than a kilometer from his workplace (according to Google Maps) and is a founding member of the Diogenes Club situated across his residence.
The club as described by Sherlock Holmes in The Adventure of the Greek Interpreter: There are many men in London, you know, who, some from shyness, some from misanthropy , have no wish for the company of their fellows. Yet they are not averse to comfortable chairs and the latest periodicals. It is for the convenience of these that the Diogenes Club was started, and it now contains the most unsociable and unclubbable men in town. No member is permitted to take the least notice of any other one. Save in the Stranger's Room, no talking is, under any circumstances, allowed, and three offences, if brought to the notice of the committee, render the talker liable to expulsion. My brother was one of the founders, and I have myself found it a very soothing atmosphere.
Mycroft first appeared in The Adventure of the Greek Interpreter, where Sherlock and Watson meet him at the Diogenes Club and its quickly made known to the reader that despite his unimpressive exterior and boring profession - the government cannot function without him. Mycroft's specialty is the connections he can make between seemingly unconnected topics and advise the government on possible outcomes of actions in situations involving multiple variables, countries, technologies etc. This of course, is only made possible because of his ability to absorb, store and index information.
Mycroft appeared in four out of sixty Sherlock Holmes stories but made a disproportionately larger impact. Mycroft Holmes appears in many Sherlock Holmes films, where he has been played by acclaimed actors like Robert Morley, Charles Gray and Stephen Fry. Mycroft also has his own novels, written by authors like Kareem Abdul-Jabbar / Anna Waterhouse, Quinn Fawcett (Chelsea Quinn Yarbro & Bill Fawcett), Ray Walsh and Michael P. Hodel. He also has a role in Alan Moore's The League of Extraordinary Gentlemen.
Mycroft Holmes at the Diogenes Club, Illustrated by Sidney Paget (Wikipedia)
1927. Achille Poirot doesn't appear in any story. He finds mention in The Labours of Hercules, and Hercule impersonates him in The Big Four, but never appears in person. There is much speculation and it is almost certain that he never existed, and was a tool of Hercule Poirot to solve crimes.
As a figment of Hercule Poirot's imagination, Achille was his twin so the two would look identical, or close enough for Hercule to impersonate him. Hercule credits his brother with equal intelligence, but not as gifted in the looks department (and without that moustache).
Agatha Christie neither confirmed nor denied if the fictional character she created himself created a fictional brother, so the 'reality' of Achille Poirot is left to the reader. But why would Achille Poirot create a brother? Is it a private joke where he compares himself to Sherlock and Achille to Mycroft? Or is Achille Poirot an actual person in the Agatha Christie universe and the author herself is parodying Arthur Conan-Doyle's writing? Even the name of the book in which Achille Poirot first 'appears' - The Big Four - gives a nod to Arthur Conan-Doyle's The Sign of the Four. However, any similarity to Sherlock Holmes' stories ends there, because The Big Four was Agatha Christie testing the waters of spy fiction.
Though The Big Four had a tough act to follow as it came after the publication of The Murder of Roger Ackroyd, and hasn't had an enduring legacy, nor was it personally favoured by the author - The Big Four managed to outsell The Murder of Roger Ackroyd.
Poirot, on his brother, from The Labours of Hercules: "If I remember rightly – though my memory isn't what it was – you also had a brother called Achille, did you not?” Poirot's mind raced back over the details of Achille Poirot's career. Had all that really happened? "Only for a short space of time," he replied.
Statue of Hercule Poirot, Ellezelles, Belgium (Wikipedia)
The Jack Reacher universe opens with Reacher stepping off a Greyhound bus at Margrave, Georgia because he remembered his brother Joe telling him that blues musician Blink Blake died there. However unbeknownst to Jack - it is his brother Joe who's died there at Margrave the night before. It takes Reacher a while to figure out what happens and a mad revenge story unfolds in Killing Floor (Jack Reacher #1).
Joe worked for the Secret Service, which was under the US Treasure Department till 9/11, and came to Margave investigating a counterfeiting operation. He is as tall as Reacher but of a slighter build; Reacher describes him as a perfectionist and an academic. Both Reacher brothers love their coffee.
Though Joe Reacher dies in Jack Reacher #1 he does make recurring appearances in later novels that cover Jack Reacher's earlier years like The Enemy. The creation of Joe Reacher is another nod to Mycroft Holmes as the less adventurous brother, who is neverthless an important part of the state and government.
