Reiu Minamiya piloted Unit 04 in the anti-Nergal AG Unit, an elite military unit employing ancient robotic weapons designated "G's Frame" against the extraterrestrial threat Nergal. Her service concluded during the Jupiter Trojan campaign, where the first AG Unit sustained heavy casualties, resulting in her official status as missing-in-action. Her disappearance deeply affected her family, particularly her father, who linked her loss to the development of DG (Divine Grace) energy technology. This event triggered her younger sister, Reika Minamiya, to enlist in the AG Unit primarily to uncover the truth behind Reiu's fate. The narrative establishes that the military, not the Nergal aliens, held responsibility for the events leading to her being declared missing-in-action.