Mission: Yozakura Family, Vol. 1

Taiyo Asano is a super-shy high school student and the only person he can talk to is his childhood friend Mutsumi Yozakura. However, Mutsumi comes from a line of master spies with amazing powers. Her oldest brother, Kyoichiro, is obsessed with Mutsumi and out to kill anyone who gets remotely near her—and his next target is Taiyo! The only way for Taiyo to save himself from Kyoichiro's murderous clutches is to marry Mutsumi!

Volume 1 of the Mission: Yozakura Family manga series by Hitsuji Gondaira.