Captain Hiroshi Fukamachi commands the Japan Maritime Self-Defense Force's diesel-electric submarine *Tatsunami*. He graduated alongside Shiro Kaieda from the National Defense Academy of Japan, forging a longstanding rivalry and professional relationship. Both officers competed for command of the experimental nuclear submarine *Sea Bat*, a position ultimately awarded to Kaieda.

Fukamachi exhibits an impulsive, emotionally driven command style contrasting sharply with Kaieda's calculated methods. His conventional worldview and strict adherence to military protocol fuel his resolve to locate and apprehend Kaieda after the latter seizes the *Sea Bat*.

During the initial incident where the submarine *Yamanami* reportedly sank after colliding with an American nuclear submarine, Fukamachi grows suspicious of the official account. He meticulously analyzes sonar data from the incident, uncovering evidence that the collision was staged and the crew survived. This discovery intensifies his pursuit of Kaieda aboard the renegade submarine.

Fukamachi personally boards the *Sea Bat* (later renamed *Yamato*) during a standoff, attempting to reason with Kaieda and halt his rebellion. When diplomacy fails and the United States military attacks *Yamato*, Fukamachi and the *Tatsunami* receive orders to escort and protect the renegade vessel. In the Battle of Tokyo Bay, he positions the *Tatsunami* to defend *Yamato* against American assaults.

During the final engagement, the *Tatsunami* sustains critical damage while shielding *Yamato* from attacks during the sinking of the support vessel *Southern Cross*. The submarine begins to sink after taking these hits. Throughout these events, Fukamachi remains dedicated to preventing further escalation while grappling with the implications of Kaieda's actions for Japan's security policies and international relations.