Hiroshi Fukamachi is a central character in the series, serving as a captain in the Japan Maritime Self-Defense Force. He holds the command position aboard the diesel-electric submarine Tatsunami. His background is firmly rooted in the Japanese naval establishment, as he is a graduate of the National Defense Academy of Japan. It was during his time at the academy that he formed a significant and long-standing relationship with fellow officer Shiro Kaieda, with the two becoming both professional rivals and, in a complex way, friends.

Fukamachi's personality is defined by a command style that is notably impulsive and driven by emotion, which creates a sharp contrast with the more calculating and methodical nature of his rival, Kaieda. He possesses a conventional worldview and demonstrates a strict adherence to military protocol and the established order. These core traits fuel his primary motivation: a resolute determination to locate and apprehend Kaieda after his former classmate seizes control of a state-of-the-art nuclear submarine. He is not driven by a desire for power but by a sense of duty and a personal conviction to stop a rebellion he sees as a dangerous violation of the rules he has sworn to uphold.

His role in the story is that of the primary pursuer and a key foil to the protagonist, Shiro Kaieda. From the very beginning, Fukamachi is skeptical of the official story that Kaieda and his crew perished in a training accident. He meticulously analyzes the sonar data from the incident and uncovers evidence suggesting the collision was staged, leading him to the truth that Kaieda and his crew survived and had commandeered the advanced submarine Sea Bat. This discovery sets him on a direct collision course with his former friend.

The relationship between Fukamachi and Shiro Kaieda is the emotional and narrative core of his character. Their history as academy classmates and rivals for the same command position creates a deep, personal tension. This tension culminates in a face-to-face confrontation aboard the renegade submarine, now renamed Yamato. Fukamachi personally boards the vessel to reason with Kaieda, appealing to him as a fellow citizen of Japan. He listens to Kaieda’s grand ambition of creating a peaceful, independent nation without borders, but ultimately rejects this vision, finding it impossible to support. This meeting solidifies their ideological divide, transforming their rivalry into a direct opposition.

Over the course of the narrative, Fukamachi undergoes a significant development. While he begins as a determined hunter of a traitor, his mission shifts in a dramatic fashion. When diplomatic efforts fail and the United States military launches an all-out attack on the Yamato, Fukamachi and the Tatsunami receive orders to change course. Instead of capturing the renegade vessel, his mission becomes one of escort and protection. This forces him into a precarious position, defending the very man he was hunting from a superior foreign force. During the climactic Battle of Tokyo Bay, he demonstrates his ultimate loyalty to his orders and his own sense of duty by positioning the Tatsunami to shield the Yamato from American assaults. In the final engagement, the Tatsunami sustains critical damage while protecting the Yamato, and the submarine begins to sink as a direct result of its defensive actions. This arc transforms him from a by-the-book officer into a man who must navigate the blurred lines between duty, country, and the actions of a renegade he once called a friend.

In terms of notable abilities, Fukamachi is a highly skilled and competent submarine commander. His primary talents lie in his tactical analysis and his willingness to challenge official accounts by investigating technical data, as shown when he used sonar records to uncover the truth of the initial incident. He is also a capable field commander, effectively leading the Tatsunami through intense combat situations, including evading anti-submarine missiles and engaging in underwater maneuvers against American naval forces.