Monsieur Rozier is a French police commissioner who helps trace Suzanne LeGros and fills in this part of the history of the corpse. He has always had some suspicions about "Jean LeGros" and his origins. He makes friends readily with Lord Peter and shares Lord Peter's enthusiasm for good food and wine.
The name "Rozier" seems to be derived from "rozzer," which was once British slang for a policeman.