Ishii (イシイ) is a scientist who lived in an abandoned airbase. When Chito and Yuuri encounter her, she was recording results from a model airplane's test flight, falling down in the process. She enlists their help to finish building a full-scale airplane based on old recovered blueprints in an attempt to fly to another city, in exchange for repairing the girls' Kettenkrad. She gives Yuuri and Chito potatoes to eat and a bath to clean themselves in and directs them on where to find a nearby rations factory.
Ishii maintains a stoic composure. Her main motivation to get through every day is the view of a distant city on the horizon, and her desire to reach it. Ishii collected any flight records and blueprints she could find around the airbase and designed her airplane through combining several features of old and modern propeller-driven models, showing some knowledge in aeronautics, though it's not known if aeronautics was her profession. She noted some advanced models were difficult-to-impossible for her to understand.
Ishii managed to safely recover via parachute after her airplane's wing structure buckled and broke apart, about 40 seconds into the flight. Despite the failure of her project that she worked so hard on, Ishii felt blissful because she had embraced her chance of failure and let go of her worries. She slowly descended to the lowest levels of the city.