Saikano(2000)

Saishuu Heiki Kanojo

Shy Chise and nervous Shuji are high school seniors in a small town and have just started dating. But everything changes when Shuji discovers that Chise has been engineered by the Japanese Self Defense Force to transform into the Ultimate Weapon! Soon Chise is torn between realizing her potential as a devastating fighting force and her desire to share a bowl of ramen with her new boyfriend!

(Source: Viz)