Born into Earth's poverty around 2211 to Kanae Hoshino, Tetsurō Hoshino's father remains largely unknown; some accounts name him as the mechanized Black Knight Faust, who recognized Tetsurō as his son before dying. Hardship defined Tetsurō's youth after his father vanished, leaving him solely dependent on his mother. Kanae met a tragic end during a Megalopolis Station blizzard, murdered by the human trophy hunter Count Mecha as she sought a pass for the Galaxy Express 999. With her dying breath, she urged Tetsurō to obtain a mechanical body for eternal life, gifting him a pendant or rocket memento. Witnessing her death ignited his thirst for vengeance and his drive to board the train.

Rescued from freezing by Maetel, a mysterious woman resembling his mother, Tetsurō accepts her offer of a free Galaxy Express 999 pass on the condition she accompanies him. Early journeys reveal a volatile personality—short-tempered, stubborn, and occasionally reckless—yet underpinned by kindness, justice, and a strong moral compass. The trauma of his mother's murder fuels lasting hostility towards mechanized humans who exploit their power, though he recognizes exceptions exist, like those mechanized under duress. His core equipment includes sunglasses from Maetel for planetary light, a durable protective cloak, and later, a hat and the highly effective "warrior's gun" (Cosmo Dragoon) gifted by an elderly woman on Titan who claimed it belonged to her son; this weapon becomes vital against mechanized threats.

Planetary stops and key relationships shape Tetsurō's development. On Titan, rescuing Maetel from bandits led by anti-mechanization fighter Antares strengthens his resolve and combat skills, with Antares advising ruthless efficiency against threats. Encounters like the glass-bodied waitress Claire, who sacrifices herself for him, and the faceless Shadow on Pluto, lamenting her lost humanity, deepen his ambivalence toward mechanization. His bond with Maetel evolves into complex familial loyalty; she acts as both guide and protector, though he questions her motives upon learning her ties to the Mechanized Empire's ruler, Queen Promethium. Crossovers with figures like Captain Harlock and Queen Emeraldas further influence his perspective. Harlock acknowledges his potential, while Emeraldas respects his selfless goals despite her own skepticism of mechanization; both see parallels between Tetsurō and their late comrade Tochiro Oyama, whose equipment he carries.

Tetsurō's climactic confrontation with Count Mecha occurs at the Time Castle, where he discovers his mother's taxidermied remains. Continuities diverge: the manga/TV series stages this battle early, while the 1979 film prolongs it, involving Antares's sacrificial aid. Achieving vengeance shifts Tetsurō's focus away from obtaining a mechanical body. His observations of mechanized societies reveal eternal life's pitfalls—complacency, loss of empathy, and existential despair—culminating in his rejection of the "free" mechanization offered at Andromeda or Planet Promethium. In the manga/TV ending, he discards the body catalog mid-journey; Maetel and the engineer Meno covertly ensure he remains human. The film depicts Promethium attempting to forcibly transform him into a screw component, thwarted when Maetel destroys her own planet to save him. Ultimately, Tetsurō embraces human vulnerability, choosing mortality and growth over stagnation, symbolizing his transition from vengeance-driven child to an autonomous individual honoring lost loved ones through lived experience.

Later narratives explore Tetsurō's adulthood, including Earth's descent into human-mechanoid war and encounters with entities like the Metanoids. His foundational traits—curiosity, resilience, and justice—persist, now channeled into broader conflicts beyond personal vengeance. Though Maetel departs to guide other youths, their bond endures as a defining influence in his journey toward self-determination.