Goro Hoshino is the father of the series protagonist, Hachirota Hoshino. A veteran engineer with decades of experience in space, Hoshino is a pioneer who was part of the missions to build the first human settlement on Mars. He has also made the journey from Earth to Mars more than five times, working as an engineer aboard spacecraft for a long period.

In the story, Hoshino serves as the Chief Engineer of the von Braun, the state-of-the-art nuclear fusion spacecraft built for a historic voyage to Jupiter. His reputation is such that he was personally scouted for this critical role by an influential magnate, which has placed him in a position of being pursued by the space defense forces.

Personality-wise, Hoshino is known as an eccentric genius. He has a playful and selfish demeanor that is often considered inappropriate for a man of his age, and he plays the part of a dirty old man who seeks to constantly irritate his eldest son. However, this behavior is generally harmless, and beneath it lies the wisdom of a veteran. When necessary, he uses his years of experience to give sound advice to his son, who strongly resembles a younger version of himself.

Despite his antics, Hoshino is highly capable. His skills as an engineer are renowned, and his experience in space is immense. His life being chased by the space defense front has also given him a unique set of practical skills, including an intimate knowledge of the ventilation system structures within the space control center and sufficient martial arts ability to hold his own in a fight against multiple opponents. Key relationships include his bond with his family, as he has a particular fondness for his wife's pork cutlet, a preference shared by his sons. His primary relationship in the narrative is with his son Hachirota, whom he goads but also subtly guides, playing a crucial role in his son's path toward joining the Jupiter mission.