Saishuu Heiki KanojoSaiKano - Takahashi, Shin

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)

Saishuu Heiki Kanojo was published in English as SaiKano by VIZ Media from June 16, 2004 to January 10, 2006.