Haruna, a Mental Model of the Fleet of Fog, serves as a sentient avatar for a battleship originally part of the Second Oriental Fleet under Hyuuga’s command. After her defeat by the Blue Steel fleet, her salvaged Super Gravitational Cannon is integrated into the submarine I-401, prompting her reassignment to port defense operations for Blue Steel. She retains her ship-based armaments and Wave Force Armor while adopting this defensive role.

Her humanoid form features blonde hair, green eyes, and a signature long coat integral to her functionality; separation from it renders her physically weakened, a vulnerability exploited in comedic or perilous scenarios. Methodical and eccentric, she devises rigorous training routines and redefines roles such as "home security specialist," interpreting the title as preparing for improbable threats through extreme measures.

In combat, Haruna partners with Kirishima, another Fog battleship Mental Model. Their synchronized strategies include merging their warship bodies into a unified entity, though attempts to physically fuse their humanoid forms result in disarray. Together, they engage in operations against Fog adversaries like the student council and assist in reactivating allied vessels.

Haruna’s evolution involves navigating human dynamics, particularly through Makie Osakabe, whose persistent removal of her coat sparks tension. Gradually, she cultivates self-awareness and tactical creativity, undertaking missions to infiltrate Fog networks and safeguard allies. Her later contributions include fortifying Iwo Jima’s defenses and aiding in reconstructing damaged Fog vessels using nanomaterial technology and adaptive engineering.