Monochrome - Teito Ninpouchou

This is a tale of modern-day magical ninjas. Reito is just an ordinary guy, who wants to maximize his own happiness by being cool and not thinking of others. However, his stupid, annoying, large-breasted neighbor Iona Shiragami keeps trying to be his friend. One day, he learns her secret: she's a ninja who guards a powerful family ninja art, and other ninja are here to take it from her. When Reito is killed in the crossfire, she uses her magical ninja art to bring him back. However, the art also turns him into a super powerful monster. Will he be able to turn back to how he was before, and what will become of his life now?

(Source: MangaHelpers)