Juuou Mujin no Fafnir

With the sudden appearance of the monsters that came to be called dragons, the world had completely changed. Before long, supernatural girls with the power of the dragons, called "D", were born within the human race. The secret existence of the only male "D", the boy Mononobe Yuu, was forcefully thrown into the school where the girls of "D" gathered - Midgard, where he saw the naked body of one of the students, Iris. Furthermore, upon meeting his long lost sister Mitsuki, Yuu seems to have been enrolled into this school...!? "When there's really no other choice, Iris, I will—kill you." "Can I really... believe you?" The curtains open as the "one and only story" about the boy who was supposed to become the strongest assassin and the poorly performing girl unfolds—! Unlimited School Battle Action!!

(Source: Baka-Tsuki)