Her Mental Model manifests as a young girl with pink hair and green eyes. She typically wears attire reflecting Yamato's fashion preferences, incorporating purple and pink hues accented with blue or yellow. This form enables both terrestrial espionage and direct naval operations.

I-400's armament includes eight frontal torpedo tubes and photon cannons for point defense. Her advanced detection systems exceed most Fog vessels, specializing in surveillance and electronic warfare. A unique capability allows her and I-402 to disrupt nanomaterial control in sister vessels like I-401, hindering emergency repairs during combat. Her Wave-Force Armor can withstand one direct hit from a heavy cruiser's Super-Graviton Cannon. Notably, she retains functional seaplanes for reconnaissance and mobility during land infiltration.

Within the *Ars Nova* anime continuity, I-400 displays heightened initiative and aggression compared to her manga portrayal. She and I-402 command the blockade at Iwo Jima, later assuming leadership of the First Oriental Fleet following Kongou's failure. They ambush and sink I-401 after its escape from Iwo Jima. When I-401 resurfaces due to Takao's intervention, I-400 and I-402 pursue but are ultimately defeated and sunk in a subsequent engagement. Her final appearance concludes with her sinking alongside I-402 during their confrontation with I-401 in *Ars Nova: Cadenza*.

Her core relationships define her actions: unwavering loyalty to commander Yamato; a tightly synchronized tactical partnership with I-402, enabled by their "joined cores" for coordinated missions; and a detached perspective toward I-401, whom she views as rebellious but refrains from engaging without explicit orders. This dynamic shifts in the anime, where she actively seeks to eliminate I-401.