Rampo Edogawa, also romanized as Ranpo Edogawa, is a core member of the Armed Detective Agency and one of the figures associated with its founding. He is regarded by his colleagues, by the police, and by himself as the greatest detective, and law enforcement regularly turns to him for help with cases that resist ordinary solutions. What makes him unusual within his organization is that he carries no supernatural ability at all; his entire reputation rests on his own intellect.

His signature method is known as Super Deduction. When he employs it, he can examine a situation and instantly determine the truth, identifying the culprit, the motive, and the location of evidence within seconds. The power appears to activate when he puts on a pair of glasses given to him by the agency's director, Fukuzawa Yukichi. In reality, the glasses function as a mental switch that places him in a calm, focused state of mind, and the deductions themselves come entirely from his extraordinary observational skills, memory, and logical reasoning. The story confirms this when it is shown that Super Deduction cannot be canceled by abilities that negate other abilities, such as Dazai Osamu's No Longer Human.

In everyday life, Rampo is confident to the point of arrogance, bold, and openly childish. He takes great pride in being called a great detective, dislikes common sense, and dismisses much ordinary knowledge as useless. He has a strong fondness for logical thinking and fantasy horror, loves sweets, and is often seen with a lollipop. His stated motto is that everything is fine as long as he is satisfied. Despite his self-centered demeanor, he struggles with simple daily tasks such as changing trains and depends on the people around him for such matters. His motivations center on solving mysteries and proving his brilliance, partly for the benefit of the agency and partly for his own amusement, while his loyalty to Fukuzawa gives his talents a deeper purpose.

His background explains much of his character. He was born the son of a famous detective known for his keen insight, but his parents died in an accident before he reached adulthood. As an orphan, his exceptional intelligence isolated him from others; he could perceive things that no one else could and suffered because he could not understand why. He was eventually taken in by Fukuzawa Yukichi in Yokohama. To protect the troubled boy, Fukuzawa told him that his gift was a supernatural ability, a kind lie that allowed Rampo to accept himself and find a place in the world. The glasses came from this arrangement, and the trust between the two men later became the foundation of the Armed Detective Agency, which was established in part to put Rampo's mind to work.

Within the story, Rampo serves as the agency's decisive brain. When an investigation reaches a dead end, he is the one who cuts through confusion and reveals the answer, often without even visiting the scene. The police request him by name because of his unmatched record of solved cases. During larger crises he also acts as a strategist; when the agency is framed as a criminal organization, his efforts help its scattered members reunite, and in a later conflict he personally negotiates with Bram Stoker to bring an end to a worldwide catastrophe.

His relationships are central to his development. The deepest is his bond with Fukuzawa Yukichi, whom he trusts completely and whose approval he seeks; Fukuzawa, in turn, sees Rampo as someone worth protecting and building an organization around. With Dazai Osamu he shares a light rivalry and mutual respect. Edgar Allan Poe, a writer from the overseas organization known as the Guild, initially resents Rampo's fame and challenges him as a rival. During their confrontation, Rampo finally admits that he does not possess any real power and is simply extremely intelligent. This confession transforms Poe's jealousy into genuine respect, and Poe later comes to regard him as a friend, even writing a mystery novel with Rampo as its hero.

Another significant relationship is his connection with Atsushi Nakajima, a newer member of the agency. Although Rampo can seem dismissive and detached, his interactions with Atsushi gradually show him learning to value teamwork and to trust his comrades instead of relying solely on his own intellect.

The central arc of the character is his slow acceptance of the truth about himself. For a long time he insists that Super Deduction is a real ability and denies any evidence to the contrary. Over time, through his encounter with Poe and the support of those around him, he comes to accept that he is an ordinary human being without special powers. Rather than weakening him, this realization becomes a source of strength: his confidence shifts from a false belief to a genuine understanding of his own talent, and he continues to use that talent to protect others.

In terms of notable abilities, Rampo possesses sharp observation, near-total recall of details, and the capacity to connect seemingly unrelated pieces of evidence into a single coherent explanation. His deduction works so quickly that it resembles supernatural insight, and because it is purely a product of human intelligence, it remains effective against those who rely on abilities. At twenty-six years old, he stands as one of the most capable detectives in the setting, not despite his lack of powers, but because of what he has made of his natural gifts.