Saikano

Shuji, a high school student in a Hokkaidō coastal town, discovers his girlfriend Chise's exchange diaries at an observatory, prompting him to reflect on their relationship through flashbacks. Their romance begins awkwardly when the shy Chise confesses her feelings, though neither possesses the emotional maturity to navigate the relationship effectively.

During a routine trip to Sapporo, enemy bombers attack the city. Amid the chaos, Shuji witnesses a small flying object intercepting the bombers before discovering Chise with mechanized wings and weaponry fused to her body. She reveals her involuntary transformation into an ultimate weapon by the Japan Self-Defense Forces, designed as Japan's last line of defense.