Motto! Kage kara Mamoru!

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)