Ranpo Edogawa, son of a renowned detective known as "The Clairvoyant" and an equally brilliant housewife, was raised by these genius parents in the countryside. After their deaths in an accident during his early teens, he moved to Yokohama per his father's suggestion and entered the police academy. Expelled for exposing the dorm master's exploits with women, he lost subsequent jobs due to uncovering corruption or discarding tasks he deemed unnecessary, like undelivered mail. His life shifted when he met Yukichi Fukuzawa during a corporate murder investigation; Ranpo deduced the secretary was the killer, impressing Fukuzawa, who took him in.

Initially believing his keen observational and deductive skills—dubbed "Super Deduction"—were ordinary adult traits, a worldview fostered by his parents' emphasis on humility, Ranpo felt intense alienation and fear interacting with others who couldn't perceive obvious truths. This culminated in a breakdown during a theater case, where he confessed terror of being surrounded by incomprehensible "monsters." To help him reconcile his abilities, Fukuzawa gave him glasses, falsely claiming they activated a supernatural skill. This lie allowed Ranpo to reframe his genius as a controllable power, becoming foundational to his confidence; he continued using the glasses as a psychological aid despite later knowing they held no power.

At 14, he became the first official member of Fukuzawa's newly founded Armed Detective Agency, created to support him. Ranpo displays extreme pride in his deductive abilities, declaring himself the "world’s greatest detective," and exhibits a childlike demeanor: openly bragging, teasing colleagues, avoiding mundane tasks, and showing laziness toward routine work. He struggles with basic activities like navigating transit, making tea, or finding his way home without assistance, relying on others for daily logistics. However, he becomes intensely serious and strategically calm during Agency crises. Deeply loyal to colleagues, viewing them as friends whose faith fuels him, he takes extreme measures to protect them—blackmailing former adversaries for aid or orchestrating complex plans to prove their innocence when framed.

His deductive skills stem solely from extraordinary intellect, solving cases in seconds through minute details and precise predictions. While he initially denied lacking supernatural abilities, he accepted this truth after the Yokohama fog incident. His limitations surface against powers altering evidence, like Mushitarō Oguri’s "Crime and Punishment," which erased crime-scene details and temporarily hindered him. He adapted by gathering circumstantial proof to expose Oguri. Evolving from a socially isolated teen, he became a leader leveraging his reputation to inspire allies, converting skeptical police during manhunts. His relationship with Fukuzawa remains central, serving as both parental figure and moral anchor.

In comedic or lighthearted portrayals, like the chibi-style spin-off, his childish traits are emphasized through exaggerated antics and chaotic interactions with colleagues and rivals like Edgar Allan Poe, though his core intellect and loyalty persist. His traumatic childhood, marked by isolation and misunderstanding, contributed to his refusal to fully adopt adult conventions. Despite occasional arrogance, he acknowledges fallibility, especially after failing to anticipate Fyodor Dostoevsky's schemes, striving to overcome weaknesses by studying adversaries' tactics. This growth is seen in his later willingness to manipulate outcomes through psychological pressure, as when coercing Oguri's cooperation by threatening to expose his late friend's crimes.