Aoi serves in the Guardwomen, a military force in a post-apocalyptic Tokyo where an unspecified catastrophe wiped out all men and many women. Under Commander Julia's command, she initially prioritizes loyalty to her comrades, survival, and maintaining order. Aoi possesses long brown hair.

Her story pivots on a profound betrayal. Julia promises Aoi the rare chance of future pregnancy—a coveted hope against humanity's impending extinction. This offer clashes with factions like Kisaragi who accept doom. Driven by desperate hope for a personal future beyond mere survival, Aoi abandons her allegiance to the Guardwomen.

She defects during a crucial confrontation in OVA episode "Day 2: -Rule-", siding with Julia against her former allies. This betrayal deepens fractures within the Guardwomen, severely impacting Captain Hitomi Landsknecht, already burdened by leadership and insomnia. Aoi's actions fuel escalating violence and moral ambiguity.

In "Day 3: -Answer-", the fallout intensifies. Her defection worsens internal divisions, forcing survivors to confront the harsh costs of genetic experimentation and biological engineering, humanity's last survival efforts. While Aoi's ultimate fate remains unknown, her choice starkly illustrates how desperation reshapes loyalty and morality in a dying world.