Hachirota Hoshino, almost universally called by the nickname Hachimaki, is the central protagonist. A Japanese man in his mid-twenties, he works as a debris collector for the Technora Corporation's Debris Section, a job viewed by many as the lowest rung of the corporate ladder. His nickname comes from the traditional Japanese headband he always wears during extravehicular activities, a symbol of his focus and determination.

Hachimaki is characterized by a brash, hotheaded, and often cynical exterior, which frequently leads to clashes with others, especially the more idealistic new recruits. He has difficulty expressing himself, particularly in romantic matters, and can be stubborn and risk-taking. Despite his abrasive demeanor, he possesses a strong sense of responsibility and a hidden core of kindness. A deep-seated ambition drives him: to own his own spaceship and explore the vastness of space. This dream, which seems impossibly expensive and distant for someone in his position, is a major source of his dissatisfaction and motivation, leading him to see his debris collection work not as a career, but as a necessary stepping stone.

His role within the story is primarily defined by his work on the debris collection ship, the Toy Box, and his journey to join a historic, crewed mission to Jupiter aboard the spaceship von Braun. Initially, he serves as a reluctant mentor to Ai Tanabe, a new member of the Debris Section. Their relationship, which begins with mutual frustration, evolves into a deep romantic connection and eventual marriage, making Tanabe the most significant relationship in his life. Other key relationships include his complicated dynamic with his father, Goro Hoshino, a renowned but eccentric engineer, and his younger brother Kyutaro, an amateur rocket maker who dismisses Hachimaki's job as insincere. His mentor, veteran astronaut Gigalt Gangaragash, is a formative figure who gave him his nickname and whose death profoundly affects him.

Hachimaki undergoes significant development throughout the narrative. His ambition to join the Jupiter mission becomes an obsession, leading him to quit the Debris Section and adopt a dangerously selfish mindset. After a near-fatal accident during an EVA, he experiences a psychological crisis known as "space loss," which evolves into an intense desire to push further into the cosmos. His single-minded pursuit of the von Braun mission causes him to alienate his loved ones, and he nearly commits a violent act of terrorism in his zeal. It is only through a subsequent accident that he has a powerful realization that he is not, in fact, alone, finally breaking through his emotional isolation and allowing him to reconcile his ambitions with his personal connections. In the end, he achieves his goal, joining the von Braun on its voyage to Jupiter, but not before marrying Ai Tanabe, thus carrying his relationships with him into the future.

In terms of abilities, Hachimaki is an extraordinarily skilled and highly qualified EVA specialist, possessing a license and qualifications that place him at the top of his field. His advanced spacewalking skills are frequently demonstrated, as is his quick thinking under pressure, such as when he creates an artificial decompression shock to subdue terrorists. His piloting and engineering skills are also noted, making him a highly competent astronaut and a formidable candidate for the rigorous Jupiter mission selection process.