Tetsurō Okino designs and pilots mechs, having developed the Bullbuster robot at Kanie Technologies prior to his transfer to Namidome Industries. At Namidome, he trains pilots to operate the Bullbuster and joins field missions combating giant beasts on Ryugan Island. His initial piloting enthusiasm, shaped by mecha fiction, clashes with the company's operational realities like budget constraints and procedures. An early mistake sees him accidentally upload Bullbuster combat footage, breaching secrecy protocols and nearly triggering company-wide repercussions; this incident drives him towards resignation until senior colleagues intervene. Over time, his narrative presence shifts from a central protagonist to an ensemble role, marking his gradual integration into Namidome's team. He participates in critical operations, including an attempt to capture a live giant beast for research and a rescue mission for suspended colleague Arumi Nikaidō – deploying the instant his own suspension lifts. His technical expertise proves vital when suggesting a Namidome rebrand (though initial ideas referencing copyrighted mecha designs are rejected) and when adapting the Bullbuster for urban combat. He aids in uncovering the giant beasts' origin: mutated bacteria developed by Shiota Chemical. Following this revelation, he joins Namidome's campaign to eliminate both the beasts and the mutagenic bacteria, culminating in a direct confrontation against Shiota Chemical's corporate exploitation. The resolution includes his commitment to fight any future beast resurgence. Minimal background details note his surname references "open sea" and "field," while his given name combines "iron" and "son."