Yozakura-san Chi no Daisakusen

After losing his family in an accident, Taiyo Asano is left with only one person he can speak to comfortably: his childhood friend Mutsumi Yozakura. Staying close to her, however, exposes a secret far beyond ordinary school life. Mutsumi is the head of a family of superhuman spies, and her dangerously overprotective older brother Kyoichiro considers Taiyo a threat. When Kyoichiro turns on him, Taiyo's safest option is also the most drastic: marry Mutsumi and become family, since the Yozakuras are forbidden from killing their own. The marriage saves his life, but it also recruits a painfully shy teenager into a household of spies, where training, missions, and enemies force him to become strong enough to protect the girl who once protected him from loneliness.