Kage Kara Mamoru!

Mamoru Kagemori is a dull high school student who's neither handsome, athletic, nor intelligent. But things aren't always as they appear. He's actually the eldest son of a 400-year-old ninja clan that specializes in protecting their neighbors, the Konnyakus. And the object of Mamoru's protection is none other than their only daughter, Yuna Konnyaku, a natural-born troublemaker!

(Source: DrMaster)