UNLIMITED FAFNIR

In the "unlimited school battle action" story, the world was transformed by the sudden appearance of monsters known collectively as dragons. Before long, girls known as "D" were born with the dragons' powers. A boy named Yuu Mononobe is the world's only male "D," although his existence is kept secret. He is forced to enroll into Midgar, a school for "D" girls, where he see a female student Iris naked. Moreover, he is reunited with his long-separated little sister Mitsuki. (Source: Anime News Network) — Description from AniList community (anilist.co).