Gunzo Chihaya The protagonist of this work. 18 years old. He was a cadet at the Marine Technology Comprehensive Academy, but two years before the start of the story, he met Iona, who had infiltrated the academy as a student. Defying government orders, the two set out together, and he became the captain of the I-401. Since then, he has been feared and nicknamed by hostile forces as both "The One Who Holds the Route" and "The Battleship Killer." He excels at situational analysis, such as sensing attacks from Takao just before they happen. His grades at the academy were 2nd overall (Kyohei calls him "Eternal Second Place"). He is always calm and collected, possessing the courage to deal with high-ranking government officials on equal terms despite his youth, and he immediately devises precise tactics with his brilliant mind and quick wit. His attitude is enough to fascinate even enemies like Takao, but he still retains a youthful vulnerability, such as becoming emotional in his room after losing to Zordan. He received a request from Undersecretary Kamikage to sink a "Fog" cruiser deployed off the coast of Kashima. He was tasked with transporting the new weapon, the Vibration Torpedo, to the United States. Observing that many "Fog" vessels have begun to possess Mental Models, he believes that communication through these models is possible, and that this fact is the key to changing the world. In the battle against the Black Fleet, he carries out a strategy using himself as bait, engaging in combat while aboard an option ship centered on the Matsushima, equipped with a Super-Gravity Cannon. Although the operation is successful, an attack fired by Kongou just before being sunk hits the Matsushima directly. Afterward, he is recovered from the Matsushima as it is rammed by the bow-only remains of Kongou. After a brief conversation, he is placed in a medical capsule installed on Hashirajima to sleep while his injured body is repaired.

Iona (I-401) A vessel formerly belonging to the "Fleet of Fog," modeled after the Imperial Japanese Navy's I-400 class submarine, the I-401. She says she received the name "Iona" from humans. She once infiltrated the Marine Technology Comprehensive Academy as a student under the name "Iona Kirino," and the data from this period was used by Takao and I-402 to construct behavioral patterns for contacting humans. Currently, she recognizes Gunzo and the others as her crew by her own will and acts according to their instructions and operations. Through refitting by Hyuga, she is equipped with a Super-Gravity Cannon, which is not normally installed on "Fog" submarines. However, in exchange for combat capability, she sacrificed the battleship-class detection capabilities that her sister ships I-400 and I-402 possess, which caused a delay in searching for Takao. After the abandonment of Iwo Jima, Hyuga joined as a sub-processor for support, and Iona's combat capabilities have increased dramatically through the use of the Mikasa Tactical System, employing option ships such as the super-heavy artillery ships Itsukushima and Hashidate, and their core/supply ship Matsushima. These option ships were created by Hyuga based on her own flagship equipment; unlike conventional Fog ships, they are hybrid vessels using nanomaterials only for critical sections (I-401 itself has replaced parts that can be substituted with human materials to reduce the consumption of hard-to-supply nanomaterials). Furthermore, she carries three Seiran aircraft, which the Fog has stopped using. Her Mental Model, Iona, takes the form of a small girl with silver hair, usually wearing casual clothes like T-shirts and shorts (after entering battle with Kongou's fleet, she wears a T-shirt that says "Eternal Second Place Proxy," perhaps because Gunzo is acting separately). She is a taciturn type who doesn't speak much beyond what is necessary, but she isn't unfriendly, sometimes using playful phrasing or showing goofy "gag faces." She actually has quite a personality; according to I-402, she is just as much of a tsundere as Takao. Depending on her computational capacity, if extra hands are needed, she can manifest small, chibi-style avatars with numbers on their foreheads to perform tasks. Among the "Fog" ships with Mental Models, she is one of the oldest, alongside Yamato. In the battle against Kongou, she entrusted all hull control to the crew and concentrated on controlling the Seiran, showing her strong trust in the I-401 crew, including Gunzo. The fact that she remained connected to the tactical network even after leaving the Fleet of Fog was a mystery to both the Fog and herself. However, during a mental world conversation with an entity calling itself "something like a self-diagnostic program" while she was asleep for a two-hour system check (the program called it "something like a dream"), it was revealed that Iona herself is the tactical network, and thus it is impossible to disconnect herself. During that conversation, Iona's appearance—hair color and clothing—was completely different from her usual Mental Model. According to the self-diagnostic program, no one, including the surface Iona herself, has noticed this Iona yet. The fact that the surface Iona knows nothing of the network issue or this other self is something she set for herself, and it is proof that she is functioning normally and perfectly. New mysteries regarding Iona, including the "self-diagnostic program," were depicted. However, upon waking, Iona had no memory of this conversation; though she felt as if something had happened, the log only recorded a system check.