This guy isn't a full-fledged character either. We think he's worth including here, though, because he's the only person in the "The Murders in the Rue Morgue" besides Dupin and the narrator who gets to speak his own lines. Even the witnesses only get their testimony reported in the newspaper – they don't even get complete sentences. So we take note in this story when someone speaks.
The sailor steps in because even Dupin's amazing logical ability has to hit a wall at some point. Dupin finds a bit of ribbon near the lightning rod. He has decided, the killer(s) used to climb up to the death house's fourth story window. This ribbon is slightly greasy (ick), from which Dupin deduces that it belongs to a sailor who uses it to tie back his long braid (or "queue," as Poe calls it). Furthermore, this guy must be sailing on a Maltese ship (Malta is an island in the Mediterranean Sea) because the knot in the ribbon is unique to Malta. And Dupin deduces that the sailor has to be a Frenchman by birth, because all the witnesses agreed that one of the two voices they heard overhead before they broke in to Madame L'Espanaye's apartment was that of a French man.
Dupin guesses that this French sailor on a Maltese ship must have witnessed these killings and has since failed to tell anyone what he's seen because he didn't want to get blamed for any part in the whole disaster. So Dupin lures the guy to his home, where he appears, muscular and mustachioed. Once the sailor is questioned by Dupin we get to hear the one bit of information that Dupin can guess at but can't prove: how the Ourang-Outang got the Madame L'Espanaye's apartment in the first place.
We don't think that we're meant to be entirely impressed with the sailor. The sailor's afraid of being blamed for something he didn't do and he's also kind of greedy and careless (what with wanting to sell the ape, and then leaving it locked in a closet and then going out drinking, only to return to find that it's escaped). He gives the impression of great strength, but bring up the Ourang-Outang and he deflates like the Goodyear Blimp after the Super Bowl. And when push comes to shove, he does the right thing: he finally decides to "make a clean breast if [he dies] for it" (116).