We're not knocking on Poe's genius by admitting that Dupin isn't fully fleshed out as a character. After all, he's not really meant to be a rounded character. He's most notable for the role he plays in "The Murders in the Rue Morgue," for taking the many clues presented during the course of the story and assembling them into a whole. Remember, "The Murders in the Rue Morgue" is a new kind of story, less interested in depth of character than in engaging the reader in logic games. (For more on this, see "In a Nutshell").
In other words, "The Murders in the Rue Morgue" is like a database of evidence – the three silver spoons, the twenty or thirty blood-smeared grey hairs, the windows locked from the inside – and Dupin is like the search engine that picks out what we need to know when we can't figure it out for ourselves. The thing about search engines is, they're not exactly social – we can't remember the last time we had a conversation with Google – and Dupin's no exception. Most of the details Poe gives us about Dupin's background and interests seem designed to show us that he's not great at playing with others.
Dupin comes from a great family that's fallen on hard times. This means that he has a private income, but still has to live on a tight budget. As a result, he doesn't have to work for a living, but also doesn't have the social or political obligations of someone from an aristocratic background who still has lots of money.
We learn that Dupin spends what little money he has on books – he and the narrator become friends when they're both looking for the same volume at a library in the Rue Montmartre. We get the sense the Dupin might not be the most social or extraverted of characters. Neither Dupin nor the narrator seem like they belong to a book club, in other words.
Dupin doesn't exactly hate other people – after all, he's willing to live with the narrator (and, we can't help but notice, on the narrator's dime). But what he likes, way more than directly interacting with others, is looking at them. He and the narrator never go out during the day, but they love to walk around the Parisian streets at night, checking out people around them and guessing who they are and where they're from.
Why does Dupin keep so separate from the rest of humanity? We think the key to solving this puzzle lies is in his interaction with the narrator. Consider the moments when Dupin suddenly interrupts the silence between them to say exactly what the narrator's thinking, out of the blue. He loves watching faces and body language to make deductions, and he values the surfaces of things as the best source of clues for what's going underneath them.
It's this trait that makes Dupin a great detective. He can look beyond obvious clues that confuse the police (like the four thousand francs on the floor, which the police think have to provide the motive, because who doesn't want lots of money? As we learn, an orangutan, that's who). He sees the small details that indicate what really happened – like the broken nail in the second window frame. But it also makes him hard to deal with as a friend: how would you feel if your best friends kept staring at your face, trying to figure out your feelings, instead of just, you know, asking? He's mostly interested in games, puzzles, and philosophizing – and not so much in interpersonal bonding.
By the way, a last word about Dupin's character: he sneers that the Prefect of Police is "all head and no body" (123) in his reasoning, that he lacks creativity even though he's got plenty of "cunning." But Dupin himself seems to be all brain and no heart – he only gets involved in trying to get Le Bon off the hook because he owes him a favor, not because the guy is innocent. Where's all of his passion, rage, what have you? Well – maybe in the Ourang-Outang? Check out our "Character Analysis" of the Ourang-Outang for more on our favorite ape and its relation to Dupin.