Who is Dupin?
If you are looking for a literary figure named Dupin, I tagged your question with Poe and one of his short stories entitled "The Purloined Letter." Edgar Allan Poe, while known for his Gothic contributions to American Literature, is also famous for creating the modern detective story (stories which feature an intelligent civilian who solves crime through logic and paying attention to detail all while outwitting the police).
Poe created a reoccurring character named C. Auguste Dupin. He is a wealthy Frenchman who enjoys analyzing various crimes in the comfort of his library, while the police lag several steps behind in their investigation. In "The Purloined Letter," Dupin discovers where a very important letter is being hidden by a thief and prevents far-reaching political consequences from taking place.
Poe's Dupin served as an inspiration for Sir Arthur Conan Doyle's Sherlock Holmes character, even though many readers think it is the other way around since Poe's detective stories are not as popular as his Gothic ones.
Edgar Allan Poe introduced the character Auguste Dupin in his earlier detective story, "The Murders in the Rue Morgue." In that story the narrator describes Dupin at great length and gives examples of the man's incredible analytical ability before launching into the narrative about the notorious double murders of two women in the rue Morgue in Paris.
Dupin served as a direct inspiration and model for Arthur Conan Doyle's famous consulting detective Sherlock Holmes, and Doyle acknowledged his indebtedness to Poe. Doyle created Dr. Watson to serve as the narrator in almost all of the Sherlock Holmes stories, and this was in direct imitation of the technique Poe had used with Dupin and the anonymous narrator of both his detective stories. In "The Purloined Letter" Poe seems to take it for granted that his reader is already acquainted with Dupin, and even with the narrator and Monsieur G- the Prefect of the Parisian police force, through reading "The Murders in the Rue Morgue."
Dupin also deploys his sleuthing skills in Poe's short story "The Murders of the Rue Morgue." By a simple process of observation and deduction, he solves the unsolvable. Furthermore, this tale is considered the very first detective story, well before the exploits of Maigret, Poirot or Sherlock Holmes.
Dupin also appears in "The Story of Marie Roget," published in 1865.