Sir Percy Blakeney is described as having most of the advantages that nature and society can bestow. He is tall, handsome, wealthy, good-natured, popular, and always immaculately dressed. The one gift he apparently lacks is intelligence. In fact, his persona as a fool is carefully constructed in a number of ways. He responds to conversation with a "perpetual inane laugh," which gives the impression that he does not quite understand what is going on. His lazy, glazed expression conveys the same message.
Sir Percy's conversation is as inane as his laugh. Early in the novel, he encounters the young, hot-headed Vicomte de Tournay, who attempts to challenge him to a duel:
“La! sir,” said Sir Percy at last, putting up his eye-glass and surveying the young Frenchman with undisguised wonderment, “where, in the cuckoo’s name, did you learn to speak English?”
“Monsieur!” protested the Vicomte, somewhat abashed at the way his warlike attitude had been taken by the ponderous-looking Englishman.
“I protest ’tis marvellous!” continued Sir Percy, imperturbably, “demmed marvellous! Don’t you think so, Tony—eh? I vow I can’t speak the French lingo like that. What?”
Sir Percy's foppish mannerisms complement his apparent incomprehension of what is going on around him. On this occasion, as on many others, his wife, Marguerite, is also present to act as a foil. Everyone knows that Lady Blakeney is exceptionally clever, and her intelligence highlights his stupidity. Sir Percy is estranged from his wife, whom he believes to be responsible for the execution of the Marquis de St. Cyr, and therefore hides his intelligence even from her. Lady Blakeney is therefore an unwitting accomplice in Sir Percy's pretense of stupidity, as she vents her sarcasm on him.