Himitsu no Otome ~Boku ga Onna no Ko ni Natta Wake~

One day Kazuki Kurachi, successor of a dojo and the protagonist of this story, is made to wear female clothes by his sister. While doing so he witnesses a bank robbery and saves a girl who was taken hostage. Impressed by his abilities and not seeing through his disguise, the girl makes Kurachi her bodyguard, and the protagonist starts to attend all-girls school as a female student Natsuki.