The narrator of "The Murders in the Rue Morgue" is clearly first person in the sense that he tells the story using "I" and only describes what he knows directly or hears in conversation. We don't have access to the main character, Dupin, except as the narrator describes him. On the other hand, "I" is dedicated to telling Dupin's story and not his own. This is what we mean by "peripheral." He stands to the side jacking up the suspense and delaying what we get to know when (i.e., Dupin has clearly decided that the guilty party is an Ourang-Outang before he tells the narrator and, therefore, us). There isn't a lot of meat to the character of the narrator –he's seems as though he's meant to collect and transfer information to the reader. We get into this a lot more in his "Character Analysis," so be sure to check out that section to learn more about the narrator.