Hyuuga began as a Fleet of Fog Ise-class hybrid battleship-aircraft carrier, flagship of the First Oriental Fleet. She was defeated and sunk by Gunzou Chihaya's submarine I-401 before the series' main events. Her salvaged Super-Graviton Cannon became I-401's primary weapon. After deserting the Fog, Hyuuga's Mental Model joined the Blue Steel faction, managing their underground base at Iwo Jima. There, she repurposed her former vessel's weaponry and Wave-Force Armor for base defense, displaying processing power rivaling the Fog battleship Kongou.

Her Mental Model appears as a woman in her early twenties with amber eyes and mid-back-length brown hair curling inward at the ends. She typically wears a dark sweater, lab coat, stockings, sometimes a dark-gray mini-skirt, and a monocle over her right eye. A floating robotic arm apparatus defines her appearance; it transforms into an egg-shaped protective shell, serves as a mobile platform, or shrinks to pocket size when not in use. Her personality blends intellectual curiosity with obsessive tendencies, showing a "techno-freak" passion for robotics, cybernetics, and upgrading I-401's systems. She experiments with non-Fog materials to develop mining machines and autonomous robots. She also harbors an intense fixation on Iona, whom she reveres as a "goddess of war," frequently expressing physical affection that Iona finds unsettling but tolerates due to their alliance.

Her technical expertise includes hacking Fog vessel systems for covert mission support. During Iwo Jima's defense against human forces, she deployed her robot army to assist Shizuka Hazumi. Later, she acted as I-401's co-processor in combat, managing advanced systems like the Super-Graviton Cannon. In a manga arc, Hyuuga accompanied Gunzou to salvage nanomaterials from her sunken hull at Bounomisaki. When I-401 faced imminent destruction by Kongou's fleet, Hyuuga intercepted the battleship Hiei using only her robotic shell and a salvaged turret. This confrontation resulted in mutual destruction of their Mental Models, though their Union cores survived. Hyuuga's core, retrieved by the Fog destroyer Yukikaze, then persuaded Yukikaze to search for Gunzou after his ship was damaged, showing persistent loyalty without a physical form.

Later, the supreme flagship Yamato assigned Hyuuga a new hull, appointing her flagship of the Second Oriental Fleet in Kongou's place. During the *Ars Nova Cadenza* film events, she consumed half of Iwo Jima's resources to reconstruct Takao's vessel, equipping it with drills from the base. She allied with Takao to combat the Fog's "student council" fleet, diverting enemies to aid I-401's mission against Musashi. Her relationships remain complex: she views Gunzou as a commander but also a romantic rival for Iona's attention, collaborates with Takao on shared objectives despite friction, and maintains wary respect for Yamato due to Yamato's connection to Gunzou's past. Her technological ingenuity and willingness to sacrifice for allies mark her evolution from a solitary Fog flagship to an integrated Blue Steel strategist.