Ishii is a survivor encountered by the protagonists in the upper reaches of a vast, abandoned city. She lives and works alone in an old air force base, having dedicated herself to a singular goal for what appears to be a considerable length of time. Her background is one of profound isolation, yet she is defined by a purpose she has created for herself. Possessing the skills of a mechanic and an engineer, Ishii discovered airplane blueprints scattered throughout the base and resolved to construct a functional aircraft from them, entirely on her own. Her primary motivation is to escape the decaying city, which has limited resources and will inevitably fail, by flying to another city she can see in the distance. She believes that staying would mean dying alongside the abandoned metropolis, and her project represents her only hope for survival and freedom.

In terms of personality, Ishii is composed, thoughtful, and appears perpetually calm or even sleepy, often with a gentle demeanor. She is also remarkably determined and persistent, laboring for years on her complex project without any assistance or guarantee of success. Despite her quiet nature, she is pragmatic and hospitable, offering food and a bath to the protagonists, Chito and Yuuri, in exchange for their help with repairs and construction. A defining characteristic is her philosophical acceptance of failure; when her plane inevitably breaks apart shortly after takeoff, she does not despair. Instead, she smiles as she parachutes to safety, commenting that failing at something one has worked hard for brings an unexpected feeling of carefreeness.

Ishii's role in the narrative is as a temporary companion and a mentor-like figure for the two main characters. When they arrive, their vehicle, a Kettenkrad, is broken, and Ishii agrees to fix it. In return, they help her complete the airplane. Through this interaction, she provides them not only with material aid but also with a new direction, pointing them toward a food production facility in the west to continue their journey. Her key relationship is with Chito and Yuuri, who serve as the first people she has likely interacted with in a long time. She treats them kindly, and the brief partnership allows her to finally finish her life's work. The relationship is mutually beneficial, as the girls gain warmth, sustenance, and a repaired vehicle, while Ishii gains the extra hands needed to achieve her dream.

Throughout her brief appearance, Ishii exhibits notable development. While she does not succeed in her primary objective of flying to another city, she achieves a significant personal resolution. Her character arc moves from one of solitary, desperate striving to a state of calm acceptance. The construction of the plane and the attempt itself become an end that provides meaning, more than the successful flight ever could have. She demonstrates that in a hopeless world, the act of pursuing a goal can be a fulfilling purpose in itself. Her abilities are impressive for a lone survivor; she is a self-taught aeronautical engineer and mechanic, skilled in reading complex blueprints and constructing a machine as intricate as an airplane from salvaged parts. She also possesses the practical knowledge to use a parachute and is capable of repairing advanced machinery like the Kettenkrad.