Background: Formerly the commander of the JMSDF diesel submarine Yamanami, he was appointed commander of the U.S. Navy's nuclear submarine Sea Bat. Abilities: Possesses the finest ship-handling skills in the JMSDF, combined with a cold political sense and high ideals. Actions: During sea trials, he suddenly leads a mutiny and dives into the deep. He declares the establishment of the independent nation "Yamato," armed with nuclear missiles, to the entire world.

Rank/Position: Commander (Commander, Junior Grade), JMSDF; Captain of the Tatsunami. Traits: A hot-blooded and passionate individual. He is the polar opposite of the always calm and collected protagonist, Shiro Kaieda. Relationship with Kaieda: They served on the same submarine in the past. Following a certain maritime accident, he harbors a relentless obsession and complex emotions toward Kaieda. Professional Respect: While he violently opposes Kaieda's actions and philosophy, they are rivals who mutually respect each other's exceptional abilities as commanders and tacticians. Pursuit: As Kaieda leads the independent nation Yamato (Sea Bat) in a rebellion against the world, Fukamachi relentlessly pursues him in the Tatsunami, engaging in high-stakes psychological warfare and tactical maneuvering at the bottom of the sea.

Position: Prime Minister of Japan. Role and Actions: Leads the government with a pacifist heart and a vow against war. Dealing with Yamato: Struggles and maneuvers to prevent Japan's international isolation and avoid war with the U.S. in response to the nuclear-armed independent submarine Yamato (commanded by Shiro Kaieda). Political Skill: Despite being at the mercy of events, he makes grave national decisions, such as declaring a continued alliance with Yamato through general elections and political maneuvering.

Position: President of the United States. Role: The primary political antagonist who is manipulated by the movements of the nuclear submarine Yamato led by Shiro Kaieda. Actions/Traits: Takes a hardline stance against Yamato's declaration of nuclear possession, suspecting Japan of secret nuclear development. He uses this as a pretext to plan a forceful re-occupation of Japan. Conflict with Kaieda: Violently opposes Kaieda's vision of world peace and nuclear deterrence from the perspective of national prestige and security, executing successive operations to eliminate or sink the vessel. Humanity: Father to Mars Bennett; he is portrayed in a complex position, showing his face as a family man while being forced to make cold-blooded decisions as the leader of a superpower.