Colonel Ducroix is a French soldier and member of the prestigious Legion of Honour who serves as one of the Marquis’s “seconds” (official attendants) during his trip to France. He officiates the duel between the Marquis and Gabriel Syme, and then he helps the Marquis, Syme, Dr. Bull, and the Professor escape the Secretary’s army by enlisting the help of the peasant, the innkeeper, and Dr. Renard. Like these three men, the Colonel is unfailingly honest and principled—to the point that he joins the Secretary’s army after the Secretary convinces him that the detectives are really criminals on the run.