Koyomi Akejima is the younger sister of Hiroshi Akishima (known as Taishi Akejima in Japanese). Before the 2047 alien invasion, she lived peacefully with her family, sketching in the garden during free moments. Her father, a military commander, died during the initial FOS attack, leaving her orphaned alongside Hiroshi. This tragedy forced Koyomi into a harsh reality of survival, dependent on her brother as her only remaining family.

Separated for two years while Hiroshi served in the Planetary Defense Forces Alliance, Koyomi lived independently yet maintained a strong emotional bond with him. By 2053, she remained a determined and resilient figure despite her youth. When Hiroshi contacted her before a critical mission, they reunited at a meaningful location. He urged her to evacuate to the Esperanza colony on Mars for safety, but Koyomi stood firm, refusing to leave Earth and choosing instead to stand by her brother, prioritizing their familial bond over personal security.

Her resolve proved pivotal during humanity's final counteroffensive. Instead of evacuating, she positioned herself to support Hiroshi directly. When the initial military operation faltered, she actively participated in executing a contingency strategy. Her presence and help were crucial in deploying an experimental weapon, contributing to the effort to reclaim Earth.