Hajiki Sanada is the sixteen-year-old protagonist of his story, a boy shaped by economic hardship and a deep-seated sense of responsibility. He lives with his widowed mother, Kyoko, and his younger sister, Satsuki, in Night Town, the impoverished district of a future metropolis called Unit Blue. His father, an astronaut, died in a freak accident during a space launch, leaving Hajiki as the designated man of the house. To support his family, he works as a delivery boy for the gruff Mr. Hachisuka at Hachisuka Express, often sacrificing sleep and putting himself in dangerous situations to make ends meet.

On the surface, Hajiki projects the image of a jaded and short-tempered delinquent. He frequently skips school, races the police on his skateboard or moped, and spends his free time with a circle of similarly aimless punk friends. This behavior, however, masks a core of profound loyalty and a weary acceptance of his limited prospects. While he claims to have no dreams for the future, his actions reveal a deep desire to protect his remaining family and reclaim a sense of stability, secretly saving money to buy back their old summer home. His practical, streetwise nature often clashes with the more idealistic worldviews of the other young people he encounters.

Hajiki's life is irrevocably changed when he comes into contact with a Gad, a mysterious black cube he was tasked with delivering. Triggered by his strong emotions during a violent confrontation, the Gad activates, absorbing all surrounding metal to birth a large humanoid robot known as a Techode. Hajiki names his Techode Lightning, after his deceased father’s plane. Lightning is designed for speed and melee combat, using extendable exhaust tubes from its neck for agile jumping and engines in its lower legs for high-speed dashes. The bond between Hajiki and Lightning is central to his role in the story, forcing him into a hidden world of other Techode owners and dangerous adversaries.

His primary driving motivation shifts from simple survival to navigating the consequences of possessing a Techode. He becomes a reluctant protector, not just of his family, but of his friends who are also drawn into these conflicts. He finds himself in a persistent rivalry with Katana, an older, more ruthless Techode owner whose philosophy and methods stand in stark opposition to his own. The name Hajiki itself is Japanese slang for gun, reflecting his quick, forceful, and erratic nature, a deliberate contrast to Katana, whose name evokes the precise and personal quality of a sword.

Over the course of the narrative, Hajiki forms a complicated but close-knit group with other young Techode owners. He often finds Takumi too righteous, Aiko too obsessed with friendship, and struggles to understand the earnest Arashi, who becomes his primary love interest. Despite their frequent disagreements, he grows to hold all of them close, secretly wishing they could all be on the same side. His key relationships extend to his demanding boss, Hachisuka, and serve as the core of his emotional development. Starting as a cynical and isolated punk, his journey forces him to confront his own feelings of inadequacy and rage, gradually revealing a more responsible and caring young man who is willing to make great sacrifices for the people he loves, culminating in a final, desperate mission into space to find answers about his father.