Kage Kara Mamoru! (TV Series)

Kagemori Mamoru 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, Mamori has protected the daughter, Konnyaku Yuuna, since they were in the kindergarten. When Yuuna is in trouble, Mamoru quickly changes into a ninja uniform, and becomes a competent ninja to help her.