Professor Hiroshi Ochanomizu is a central recurring character in the anime Astro Boy, appearing in every major animated version of the series as well as the original manga on which they are based. He serves as the head of the Ministry of Science, a position he takes over from the former minister Doctor Tenma, and he is the person most directly responsible for shaping Astro's life. In the original telling, Ochanomizu first notices Astro performing at a robot circus after the boy has been abandoned by his creator, recognizes that the little robot is unlike ordinary machines, and arranges to take him into his care. His physical appearance is highly distinctive: he is portly and older, with a bald head ringed by fluffy white tufts of hair and an enormously oversized nose that frequently becomes the subject of comedy, sometimes being injured or even falling off during his misadventures. The English adaptations gave him names such as Doctor Elefun as a playful reference to this prominent feature.

Ochanomizu is good-natured, gentle, and famously emotional, often moved to tears at moments of strong feeling. He is also scatterbrained and eccentric, easily excited by scientific mysteries and prone to being kidnapped or mistreated by villains despite his brilliance. His defining belief, consistent across nearly every version of the story, is that robots are not mere tools but beings capable of thought and feeling, and that humans and robots can coexist peacefully. This conviction sets him apart from most other human characters, including Doctor Tenma, and makes him a dedicated campaigner for robot rights. He is known for being the one who explains to Astro that he has a human heart and for teaching him the meaning of justice. His motivations are rooted in compassion and a sense of responsibility: he wants Astro to experience an ordinary, happy childhood while also using his abilities to help others, and he repeatedly risks his career, reputation, and personal safety to protect Astro and other robots from exploitation, fear, and cruelty.

In the story, Ochanomizu functions as Astro's guardian, mentor, and chief source of support. He provides Astro with a home, repairs him when he is damaged, helps maintain and upgrade his systems, and sends him on missions in his capacity as Minister of Science. He also acts as Astro's advocate, pleading on his behalf before authorities and defending him from those who see robots as threats. Beyond his direct care for Astro, he serves as a moral anchor in a world where robots are routinely abused, frequently opposing scientists, politicians, and military figures who promote fear and discrimination. To give Astro the family life he never had, Ochanomizu builds an entire robotic family for him, including parents, a mischievous younger sister named Uran, and an older brother named Cobalt. His role shifts somewhat between adaptations; in one animated version he is depicted as Astro's creator rather than his rescuer, while in others he assumes guardianship after Astro is abandoned.

The most important relationship in Ochanomizu's life is his bond with Astro. He acts as an adoptive father, treating Astro as if he were his own son and offering both emotional warmth and academic guidance, while Astro looks to him as a moral and scientific reference point. The relationship with Doctor Tenma is defined by contrast: the two scientists were formerly friends and colleagues, but they hold opposite views on robots, with Tenma coldly regarding them as replaceable machines and Ochanomizu regarding them as persons deserving of respect. Other robots and supporting characters frequently rely on Ochanomizu for protection, legal support, and scientific assistance, and he is regarded as a kind of grandfather figure in his community. His personal background is only lightly sketched. In some later works he is shown as a widower with a granddaughter, and in a prequel series he has a sister, while many details of his past remain unexplored. In one animated film it is mentioned that he comes from Polynesia.

Across the various versions of Astro Boy, Ochanomizu remains consistently defined by his kindness and his commitment to robot rights, though the specifics of his situation change with each adaptation. His relationship with Astro deepens as Astro moves from being a rescued circus performer to a celebrated hero, with Ochanomizu becoming the steady presence who keeps Astro grounded and reminds him of his own humanity. His emotional openness, which early stories treat partly as a comic weakness, is presented in the narrative as his defining trait, and his idealism is tested by the dangers of the world without ever being abandoned.

Ochanomizu has no superhuman powers, but he is a highly accomplished scientist with deep expertise in robotics and artificial intelligence. He creates advanced robots, including Astro's sister and parents, and he is capable of repairing, rebuilding, and upgrading Astro after battles. He is also a prolific inventor whose creations frequently play a decisive part in resolving the crises of individual episodes. His oversized nose is so sensitive that it is used as a recurring comic vulnerability, and his habit of being abducted by villains is a running joke throughout the series. In addition to his technical skill, his main abilities are his persuasive advocacy, his moral clarity, and his willingness to stand by his principles even when doing so places him in harm's way.