Mamiya is the Executive Officer (XO) of the Imperial Japanese Navy heavy cruiser "Unebi", responsible for ensuring that the Captain and First Officer's orders are carried out, managing the crew, and ensuring the well-being and battle-readiness of the entire ship.

She is a woman of few words who is effective at her job except for her habit of spacing out in daydreams, most of which center on her romantic obsession with the First Officer, Commander Kuki, the deaths of her rivals for Kuki's affections, and her performing impossible feats of daring to impress Kuki.

Mamiya wears a katana and is unparalleled at iai-nuki, the art of the fast draw. Her feats of improbably good swordswomanship include being able to slice someone's clothes off without touching their skin, being able to deflect bullets, and being able to slice clean through opponents' weapons, including swords and guns. However, in her daydreams, she slices up enemy battleships.

She becomes an implacable death machine when sufficiently angered, and the only thing that can distract her from the anger is Commander Kuki.