Hiroshi Agasa, most commonly called Professor Agasa, is an inventor who lives next door to the home of Shinichi Kudo in the Beika district of Tokyo. His name is written with the kanji for "doctor" or "professor," which is why nearly everyone addresses him as Professor rather than by his given name. He attended Teitan Elementary School and Okuho Middle School, and later completed a doctoral course in the engineering faculty of Toto University. His full name and the naming of his relatives are inspired by mystery writer Agatha Christie. He is around fifty-two to fifty-three years old, and he lives alone in a large house that doubles as a research laboratory, supported mainly by patents and inventions such as video games. He is known to have a special fondness for pasta and tomato juice from a restaurant called Colombo, and he owns a yellow Volkswagen Beetle that he uses to take the children on outings, though the car is famously unreliable.

In personality, Professor Agasa is jovial, warm, and outgoing, with many friends in his community. He is a bachelor but behaves in a very paternal way toward the children around him, especially the members of the Junior Detective League. He is highly intelligent and takes great pride in his inventions, although many of his creations are eccentric, impractical, or prone to malfunction. He also enjoys puns and wordplay, and he regularly presents the children with riddle quizzes during trips and in the theatrical films. He tends to look older than his actual age, appearing as a slightly overweight man who is bald on top, with silver hair remaining at the sides, thick eyebrows, a mustache, a somewhat bulbous nose, and round glasses, usually seen in his laboratory attire.

His motivations are rooted in friendship and a protective sense of responsibility. He dreams of becoming wealthy through his inventions, but his deeper drive is to support Shinichi and the people who rely on him, even when doing so places him in danger. He is not a fighter and does not confront criminals directly, but he willingly puts his knowledge and technical skill at the service of those he cares about, and he acts as a cautious voice of reason, especially in matters involving dangerous enemies.

His role in the story is that of a trusted ally and technical support. He is the first person to learn that Shinichi has been reduced to the body of a child and now goes by the name Conan Edogawa. It is Professor Agasa who advises Shinichi to keep his identity secret, helps him create the Conan Edogawa persona, and arranges for him to enroll at Teitan Elementary, which is how the Junior Detective League comes into being. Almost all of Conan's special gadgets are designed and built by him, including the bow-tie voice changer, the power-enhancing sneakers, the tracking glasses, the wristwatch tranquilizer gun, and the turbo-powered skateboard, along with the badges and flashlight watches used by the Junior Detective League. Because he knows Conan's identity, he sometimes serves as a stand-in speaker during deductions, delivering Conan's reasoning aloud while the others believe he is solving the case himself. He also takes in Shiho Miyano after she escapes from the Black Organization, learns her true identity, and helps her assume the name Ai Haibara, later acting as her guardian and housemate. He is among the very few people aware of the secrets surrounding Conan and Haibara, and he is entrusted with supporting their plans against the Black Organization from the sidelines. He also informs Shinichi's parents of what has happened to their son, and he is aware of the hidden identities of other allies.

His key relationships are defined by trust and care. With Shinichi and Ran, he has been a friend since their childhood, and Shinichi relies on him completely despite sometimes mocking his strange inventions. With Ai Haibara, he serves as a protective guardian, and she in turn watches over his health, imposing a strict diet to manage his weight. With the members of the Junior Detective League, Ayumi, Genta, and Mitsuhiko, he is a grandfatherlike figure who escorts them on trips, most of which unexpectedly turn into criminal cases. He maintains cordial relations with the police, particularly Inspector Megure and other detectives, who address him respectfully. He is also an old acquaintance of Shinichi's parents, Yusaku and Yukiko Kudo, and he has a gentle, respectful connection with Kogoro Mouri. One of his most personal relationships is with Fusae Campbell, his childhood first love from elementary school, whom he meets again many years later; the two still hold feelings for each other despite their long separation.

In terms of development, Professor Agasa begins as an eccentric neighbor with a reputation for building odd gadgets and gradually becomes one of the central pillars of the ongoing conflict, holding knowledge of several characters' true identities and taking part in planning against the Black Organization. His backstory is expanded through stories about his youth, his family, his friendship with Shinichi's parents, and especially his first love. He occasionally steps into a more active role when asked to present deductions, and his inventions become increasingly sophisticated and important to the plot. Over time, he also becomes more careful and protective, serving as a brake on the children's reckless behavior and offering words of encouragement or reflection to criminals he encounters during cases.

His most notable abilities lie in engineering and invention. He can produce compact, high-functioning devices that allow a child-sized detective to operate effectively in dangerous situations, and some of his creations are sophisticated enough to gain recognition from professionals and to be used during official events. He has also collaborated on major projects such as the development of a new gaming system, and his surveillance and disguise equipment has been used in complex operations involving law enforcement. Beyond his technical skills, he is remarkably lucky when it comes to winning food tickets and prizes in drawings, and his cheerful nature and gift for wordplay make him a source of lightness within the otherwise tense world of the story.