Docteur Agasa is a pivotal supporting character in the story of Detective Conan, first appearing as the initial confidant of the protagonist after a life-changing event. He is Shinichi Kudo’s next-door neighbor and a long-time family friend, making him one of the few people fully aware of Shinichi’s transformation into Conan Edogawa from the very beginning. A middle-aged man of fifty-two to fifty-three years, he is an inventor and engineer by profession, though his genius is often obscured by his eccentric and sometimes bumbling nature. Physically, he is described as slightly overweight with a balding head, the remaining hair being silver, along with thick eyebrows, a mustache, and a bulbous nose, often seen wearing round glasses.

In the crucial first episode where Shinichi is shrunk, Agasa is the first person he turns to for help. Initially disbelieving that a small child could be his brilliant teenage neighbor, Agasa is convinced when the boy uses his deductive reasoning to reveal that Agasa has just run home from a specific restaurant. This moment establishes the core dynamic of their relationship: Agasa as a trusted, if sometimes overwhelmed, adult ally who provides support for Conan's intellectual battles. His personality is jovial, caring, and outgoing, which allows him to maintain a wide circle of friends and convincingly fabricate cover stories for Conan and later Ai Haibara, simply presenting them as the children of acquaintances. Despite being a bachelor without children of his own, he possesses a strong paternal instinct and genuinely enjoys the company of young people.

His primary motivation throughout the series is to protect Conan and help him return to his original body and dismantle the Black Organization. He immediately grasps the danger Shinichi is in, warning him to keep his identity a secret for fear that the men in black will kill him and everyone he has told. He is instrumental in creating Conan’s new identity, not only by suggesting the alias "Conan Edogawa" but by constructing the secret gadgets that allow him to continue his detective work. These inventions, which are the core of his role in the story, include the voice-changing bowtie, the stun-gun wristwatch, the turbo-powered shoes for enhanced kicking strength, the detective badge communication system for the Detective Boys, and the solar-powered skateboard. While he is a brilliant scientist, many of his other inventions are unsuccessful or cause small explosions in his home laboratory.

His key relationships extend beyond Conan. He shares a close friendship with Shinichi's parents, Yusaku and Yukiko Kudo, and is entrusted with their son's secret. His most significant other relationship is with Ai Haibara, the former Black Organization member Shiho Miyano, whom he finds collapsed near his home and takes in. Their dynamic is uniquely humorous and affectionate; the much younger Haibara often acts as a caretaker for the professor, constantly monitoring his unhealthy diet, particularly his love for pasta and fatty foods, to manage his cholesterol. In return, he provides her with a safe, paternal home. His first love from elementary school, Fusae Campbell, is a recurring emotional touchstone, and he enjoys a surprising amount of luck in winning lotteries and food tickets.

Within the narrative, Agasa serves not only as the inventor and strategist but also as a stabilizing, trustworthy adult figure who provides a safe haven. He often takes Conan and the Detective Boys on trips in his distinctive yellow Volkswagen Beetle, which frequently leads them into new criminal cases. He is a man who, while not engaging in direct physical confrontation, is the backbone of Conan's support system, offering intelligence, resources, and unwavering loyalty from the very moment the great detective turned small.