Saishuu Heiki Kanojo

Shuji and Chise are two Hokkaido high school students barely starting a relationship with each other. Then, with no warning, a squadron of bombers from an unknown country destroy the city of Sapporo. All hope is lost when suddenly, the bombers are destroyed by a small red light floating across the sky. At this point, Shuji finds out that the one responsible for eliminating the enemy forces is Chise, who reveals herself as a cyborg engineered for mass-destruction.

(Source: ANN)