Koyomi Akejima, younger sister of Planetary Defense Forces Alliance soldier Taishi Akejima, emerges from the ashes of humanity’s shattered peace after the 2047 FOS invasion. The catastrophic attack obliterated major cities, claiming their father Koshiro—a high-ranking military officer—and cementing Taishi’s vengeful resolve. Left orphaned, the siblings became each other’s sole anchors in a world fractured by war.

Initially sheltered within a military academy, Koyomi’s life intertwined with the resistance’s structure until her evacuation to Mars ahead of Earth’s 2053 counteroffensive. This strategic retreat, part of a refuge initiative to safeguard civilians, positioned her as both survivor and emblem of humanity’s fragile continuity.

Her bond with Taishi strains under the weight of his consuming obsession with avenging their father. Though tethered by loyalty, she questions the cost of his singular focus, fearing its toll on his humanity. This unresolved tension fuels her clandestine return to Earth during the FOS assault on the Planetary Defense Forces’ Mount Fuji base. Defying evacuation orders, she infiltrates the conflict zone, intertwining her fate with the battle’s outcome despite avoiding direct combat.

Koyomi’s journey mirrors the duality of survival and sacrifice. Her Mars evacuation underscores desperate hopes for preservation, while her Earthbound return subtly aligns her with collective resilience. The narrative leaves her ultimate destiny unspoken, her arc concluding as the battle’s echoes fade, her legacy lingering as a bridge between the lost past and an uncertain future.