Hiro Seno serves as the narrative's central protagonist, introduced as a talented monobike racer and leader of the Killer Commandos team in Aphrodia's capital, Io. Originating from the wrong side of the tracks, his rebellious streak manifests in pre-invasion clashes with local authorities. He gains renown as a fearless hotshot in the brutal, Rollerball-inspired racing circuit, skills later proving vital in combat after the Ishtar invasion. Hiro's defiant, hot-blooded personality initially channels into apathy toward the wider conflict, prioritizing personal survival and protecting his inner circle—including girlfriend Maggy Nakamoto.

The Ishtar occupation and teammate Will’s death drive Hiro toward vengeance and responsibility, culminating in a failed guerrilla attack on an Ishtar Octopus tank at the stadium. This defeat forces Hiro and surviving Commandos into alliance with the Aphrodian Freedom Force’s Bloodhound Squadron, led by Lieutenant Kurtz. Their relationship begins antagonistically, marred by mutual disdain and conflicting war ideologies. Kurtz manipulates Hiro into joining via a perilous monobike race across a ravine; losing the race compels Hiro to accept Kurtz’s bargain—his and his friends' freedom in exchange for Bloodhound service. This pivots Hiro from reluctant survivor to active combatant.

During Io’s final assault, Hiro leverages his monobike expertise and combat instincts to corner General Donner’s command tank, exploiting Donner’s rage to destroy it through luck and skill. This act catalyzes the Ishtar forces' collapse after Donner’s demise. Post-battle, mutual respect emerges with Kurtz, who gifts Hiro his monobike. Hiro reunites with Maggy at a refugee camp, achieving closure on the conflict and personal bonds, though his long-term path remains open.

Across adaptations, Hiro’s core remains consistent. The manga depicts heavy conventional motorcycles requiring physical strength, while the anime film uses challenging monobikes with magnetic railguns. The manga shows Aphrodia’s defeat after initial successes; the film ends in victory following Donner’s death. Dark Horse Comics’ manga release renames Hiro "Ken." His visual design echoes Yoshikazu Yasuhiko’s character Arion.