Hiroshi Agasa, commonly called Professor Agasa or Dr. Agasa, is a pivotal supporting character known for being a genius inventor and a trusted ally to the main protagonist. He is a 52-year-old man who holds a doctorate in engineering and lives a comfortable life as an inventor in a large house located in Tokyo's Beika neighborhood, where he is also the next-door neighbor and a long-time family friend of the Kudo family. Having known Shinichi Kudo and Ran Mouri since their childhoods, he occupies a unique position as a respected elder and guardian figure.

Agasa’s role in the story begins with a dramatic discovery when a heartbroken and shrunken Shinichi appears at his doorstep. He is the very first person to learn that Conan Edogawa is, in fact, the teenager Shinichi Kudo after being poisoned by a mysterious drug. From this moment, he becomes an invaluable confidant and strategist. Upon learning the truth, Agasa wisely advises Shinichi to keep his identity a secret to protect those close to him from the dangers of the Black Organization. He then actively helps construct the new identity of Conan Edogawa, going so far as to enroll him in Teitan Elementary School, which leads to Conan meeting the other children who form the Junior Detective League.

Beyond being a strategist, Agasa's primary contribution comes from his inventive genius. He is the creator of all of Conan’s essential gadgets that enable his detective work. These inventions include the voice-changing bow tie, which allows Conan to mimic the voice of the sleeping detective Kogoro Mouri, the power-enhancing kick shoes, the tranquilizer dart watch, and the太阳能-powered skateboard. For the Junior Detective League, he also created the detective badges that function as long-range communicators and the flashlight-watches. While he proudly styles himself as a genius inventor, his creations are not always successful; his home laboratory is prone to occasional small explosions from experiments gone wrong. His income largely comes from patents for video games, which he develops and for which the Detective Boys serve as enthusiastic test players.

Agasa's personality is jovial, outgoing, and somewhat easygoing. He has a particular fondness for pasta and tomato juice and is known for his cheerful demeanor. However, he is also deeply paternal and protective, viewing the children around him almost like family. This extends to his living situation, as he has taken in Shiho Miyano, the inventor of the poison that shrank Shinichi who escaped from the Black Organization under the new identity of Ai Haibara. Their cohabitation has created one of the most unique and humorous relationships in the series. Despite their actual ages, Haibara often acts as the more responsible figure, constantly monitoring Agasa’s diet and health, removing fatty foods from his reach in an attempt to manage his health. This dynamic, where a young girl mothers an elderly professor, is a recurring source of comedy and shows their deep bond; he saved her life by taking her in, and she cares for his well-being in return. It is also later revealed that Agasa had known Haibara's parents personally, adding a deeper layer to his role as her guardian.

Throughout the story, Agasa provides a stable base of operations. He often takes the Junior Detective League on camping trips and outings in his unreliable yellow Volkswagen Beetle, which has a famous tendency to break down in the middle of journeys, often leading the group into unforeseen criminal cases. He is also a key figure in the ongoing effort to conceal the truth from the Black Organization, working alongside Conan's parents, Yusaku and Yukiko Kudo, to develop plans and provide support. While he has lost touch with some childhood acquaintances, such as his first love Fusae Kinoshita, his primary life is consumed by his role as the inventive and paternal anchor for Conan, Haibara, and the young detectives, making him far more than just an eccentric neighbor.