The famous Belgian (though he's often mistaken for French more than once) Detective. Not much is known of his life (he had a habit of supplying false information about that particular subject) before he joined the Belgian police force. But even then, his time in the force is sketchy, with only his friend Inspector Japp giving some solid insight into (as they both worked together at that point in time).
Whatever the truth, he was forced out of Belgium by WWI, where he became a refugee in Britain, where he worked as a secret service agent (his keen investigative abilities were well known in Belgium, and it is through this reputation that the British Secret Service came to know of him). His most well-known achievement during this time is when he foiled a plan to abduct the Prime Minister.
After the War he became a free agent, undertaking various civilian cases. Setting up shop at 56B Whitehaven Mansions, Charterhouse Square in Smithfield London.
During this time he travelled all over Europe and the Middle East, solving cases wherever he could find them (or stumble upon). This was his most active period in his career, and the time period when he solved the Murder on the Orient Express, (perhaps his most well-known case).