Kokoa is Subject 22 of the Watatsumi series, one of nine genetically identical modified magicians created in an experimental program at a National Defense Navy research facility on Minamitate Island. The Watatsumi subjects were incorporated as components within a large-scale CAD machine designed to activate the strategic-class magic sequence "Meteorite Fall," which manipulates astronomical objects to collide with Earth as weapons. Kokoa fled the facility with the aid of Dr. Morinaga, a project researcher who became her ally.

She possesses black hair, blue eyes, stands 1.40 meters tall, and weighs 34 kilograms, matching the appearance of all Watatsumi subjects. After escaping, Kokoa took refuge near a villa on the Ogasawara Islands during the Shiba siblings' spring break. There, she encountered Miyuki Shiba and her friends, disclosed her status as a deserter from the naval base, and explicitly pleaded for their help to rescue the eight remaining Watatsumi subjects imprisoned at the Minamitate facility.

Kokoa directly instigated the infiltration mission into the facility. While she did not engage in combat, her background knowledge proved vital in exposing the Meteorite Fall experiment's purpose and mechanics. Following the successful liberation of the Watatsumi series and the facility's destruction, Kokoa and the other rescued subjects were relocated to the Kanto branch of the Magic Association. Her story concludes with this reunion under the Association's protection, alongside allies Mayumi Saegusa and Mari Watanabe.