Guardian Ninja Mamoru

Mamoru Kagemori is a nerdy-looking boy whose hair is disheveled and who wears thick glasses. But he is actually the scion of a ninja family, who has secretly protected their neighbors, the Konnyaku family, for 400 years. In particular, Mamoru has protected the daughter, Yūna Konnyaku, since they were in kindergarten. When Yūna is in trouble, Mamoru quickly changes into a ninja uniform, and becomes a competent ninja to help her.

Synopsis from Anime News Network