He began as a bet in 1916 when Agatha Christie (working for the Voluntary Aid Detachment in Torquay, providing nursing care for military personnel injured in the war) was challenged by a friend to write a detective story in which the reader would not be able to spot the criminal.
Christie, 26 at the time, loved detective fiction, particularly Sherlock Holmes, but wanted to create a detective who relied on psychology to catch criminals rather than just the hard evidence of a trail of clues which the reader could follow for themselves.
The result of the bet was The Mysterious Affair at Styles and Hercule Poirot: "He was hardly more than five feet four inches but carried himself with a great dignity. His head was exactly the shape of an egg, and he always perched it a little on one side...The neatness of his attire was almost incredible; I believe a speck of dust would have caused him more pain than a bullet wound.'"
A prissy Belgian who once complained about the failure of chickens to lay exactly the same size of egg - he liked symmetry at the breakfast table - and spent considerable time on his hair and moustache seemed an unlikely hero; and yet, 100 years after the publication of his first case, Hercule Poirot stands shoulder to shoulder (albeit he needs to stand on a box to do so) with Holmes as one of the immortals of literature.
That first book, rejected by a number of editors and companies before it was published in New York (she accepted a £25 fee and no control of copyright) in October 1920 was a critical rather than commercial success. But this was a time when newspaper critics still had a huge influence on the reading public and when what later became known as the post-war 'Golden Age of Detective Fiction' was taking root. By the time of his third case, The Murder of Roger Ackroyd, published in 1926 (and which, very controversially, broke one of the 10 rules of the whodunnit genre), Poirot was a success. So successful, in fact, that Christie named her house Styles.
Christie did borrow one thing from Sherlock Holmes, with Captain Hastings and Inspector Japp serving as her version of Dr Watson and Inspector Lestrade. But Hastings never caught the public imagination in the way that Watson did. Watson was there entirely for the benefit of the reader, asking the questions that popped into our minds as Holmes delivered the dazzling displays of deduction.
Watson was everyman, willing to carry the load for our collective stupidity in being able to see without actually observing. But for most of the time Hastings was just a vehicle for pushing the plot along; a role that, in a number of the novels and short stories, was taken over by other characters.
As with Holmes though, it quickly became apparent that the character was bigger than the plot; the sort of character that actors wanted to play. The first to portray Poirot was Charles Laughton, in a 1928 adaptation of The Murder of Roger Ackroyd called Alibi, which played in both the West End and on Broadway. Laughton was an unlikely choice, better known for his barnstorming theatrical style (think Captain Bligh in his Mutiny on the Bounty) than for the quiet fussiness of the detective. But audiences, many of whom probably hadn't read any Christie, came for Laughton and left with a liking for the character. In 1931 Claude Austin Trevor Schilsky (born in Belfast in October 1897 and known by his stage name Austin Trevor) became the first actor to play Poirot on screen, in the film version of Alibi. He played him a few more times in the Thirties, but without much success. Eighty-six years on, in 2017, another Belfast-born actor, Kenneth Branagh, played Poirot in a hugely expensive version of Murder on the Orient Express. Branagh returns to the role later this year with a version of Death on the Nile.
Like Laughton, Branagh played the character his own way and not particularly faithful to Christie's idea of Poirot. Interestingly, there are also some lovely touches of Poirot in Daniel Craig's portrayal of private detective Benoit Blanc in the recent film Knives Out, a delicious pastiche of big-house murder mysteries.
In the same way that Holmes fans have their favourites - Basil Rathbone, Peter Cushing, Jeremy Brett (my 'ideal' Holmes) and Benedict Cumberbatch - Poirot fans have theirs, too.
The list of films and actors is nowhere as long as the Holmes list, but along with Laughton and Branagh it also includes John Malkovich, Robert Powell, Ian Holm, Orson Welles, Albert Finney, John Moffatt (the best radio Poirot, so far), Tom Conti and Tony Randall. Two, of course, stand out: Peter Ustinov and David Suchet.
Ustinov played him six times between 1978 and 1988; three big-screen, star-studded, enormously successful feature films (Death on the Nile, the best of them) and then three made-for-television versions. In one of those, Thirteen at Dinner, David Suchet played Inspector Japp. Rosalind Hicks, Agatha Christie's daughter, observed Ustinov during a rehearsal and commented: "That's not Poirot! He isn't at all like that!" Ustinov overheard and replied: "He is now!"
But the Poirot shoes that all future actors must try and fill belong to David Suchet. For a generation he is Hercule Poirot, their Hercule Poirot.
More important - like Jeremy Brett as Sherlock Holmes - he is the Poirot as written by Agatha Christie. Read again that description of Poirot at the top of this piece and it is Suchet you will see. Suchet, who played him between 1989 and 2013, is the only actor to have completed the entire Poirot catalogue, including 33 novels and dozens of short stories.
His final interview when the series ended summed it up perfectly: "Poirot's death was the end of a long journey for me. I had only ever wanted to play Dame Agatha's true Poirot... He was as real to me as he had been to her: a great detective, a remarkable man, if, perhaps, just now and then, a little irritating. I think back to Poirot's last words in the scene before he dies. That second 'Cher ami' was for someone other than Hastings. It was for my dear, dear friend Poirot. I was saying goodbye to him as well - and I felt it with all my heart."
In 1944 Raymond Chandler described the ideal fictional detective as a man comfortable on mean streets, but "who is not himself mean, who is neither tarnished nor afraid... He has a sense of character, or he would not know his job. He will take no man's money dishonestly and no man's insolence without a due and dispassionate revenge. He is a lonely man and his pride is that you will treat him as a proud man." That's Poirot.
And that's why Poirot will live on when so many other fictional characters have come and gone, rattling through our memories when we set down the book, switch off the television or radio and wait for the next distraction. Christie, like Conan Doyle, wasn't the most revered of the Golden Age writers and some of her work looks cumbersome and contrived nowadays. The age of gathering together a number of witnesses and suspects in a country house, or hotel, or train, or boat has disappeared.
The era of the amateur consulting detective has long gone, too. But, like Holmes, Poirot was a big enough character to escape from those confines.
He remains believable and likeable, irrespective of time and location. Those 'little grey cells' are still capable of dealing with any problem, because Poirot was always more interested in people than in clues.
A new generation of authors, including Anthony Horowitz and Sophie Hannah, has been inspired by Christie and Poirot - pastiche is a booming business. Kenneth Branagh plans a few more Poirot films.
The books have never been out of print. New radio and TV versions are in the pipeline.
A century on and the brand is huge. There will always be another case demanding his attention: "For somewhere," said Poirot, indulging in an absolute riot of mixed metaphors, "there is in the hay a needle, and among the sleeping dogs there is one on whom I shall put my foot, and by shooting the arrows into the air, one will come down and hit a glass house!"