Airi, a wealthy girl, is a classmate and friend of Mamoru and Yuna. She discourages any relationship between Yuna and Mamoru, because childhood friends who become lovers are generally unsuccessful. However, she also has feelings for Kagemori.    (Taken from Wikipedia)

Kagemori Mamoru is a dowdy boy whose hair is disheveled and who wears thick glasses, but he is actually the son of a ninja family. They have protected their neighbors, the Konnyaku family, secretly for 400 years. Mamoru has personally protected their daughter, Konnyaku Yuuna, since they were in kindergarten. When Yuuna is in trouble, Mamoru puts on ninja suit and turns into a competent ninja to help her.

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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)

Mamoru Kagemori is a high school student who is neither good-looking, studious nor athletic. However, he is actually a skilled ninja who secretly protects his neighbour, Yuuna Konnyaku.
 
Mamoru and Yuuna are now third year students, but their peaceful times are suddenly interrupted on the day of the opening ceremony when Marimo, a girl who claims to be Mamoru's younger sister, transfers to his school. Marimo immediately issues a challenge to Mamoru, intending to take his duty of protecting Yuuna away from him!
 
A new chapter of the popular ninja romantic comedy has started! 
(Source: Kadokawa, translated)

Airi Sawagashi