What is the name of Hercule Poirot's supposed twin brother in 'The Big Four'?

What is the name of Hercule Poirot’s supposed twin brother in The Big Four? The answer is Achille Poirot.

Achille appears in Agatha Christie’s 1927 novel The Big Four, an unusually international espionage thriller involving a criminal organisation bent on global power. Hercule claims that Achille is his twin brother, but the supposed sibling is ultimately revealed to be Poirot himself in disguise.

The ruse helps Poirot operate secretly after staging his own death. Achille is presented as physically similar to Hercule but without the famous moustache, allowing Poirot to create the impression that someone else has taken his place. The name also recalls Sherlock Holmes’s brother Mycroft, though Achille is a fabricated identity rather than a genuine relative.

The Big Four began as a group of shorter stories that Christie reshaped into a novel. Its episodic structure and melodramatic conspiracy make it quite different from the more conventional country-house mysteries associated with Poirot.