From MangaHelpers:

Humanity was once menaced by black fairies, but they were fought by the black fairy hunters who eventually sealed their ruler away. Yamamoto Ryuu lives what seems to be an ordinary human high school life, hanging out with his childhood friend, Yuki, but when he starts to really think about the details of his existence, its reality doesn't hold together. He begins to remember who he really is-- the black fairy hunter Dragon-- and how he ended up in his current predicament. He had been going after a black fairy called Scheherazade, who placed him in this mysterious "Yuki"'s dream. Scheherazade claims that Yuki is actually the black fairy ruler, Black Butterfly, and she needs Ryuu's help to keep other black fairies from waking her up from her idyllic dream. As he finds himself going along with these fairies, he remembers even more troubling things, such as the fact that Yuki looks suspiciously similar to his little sister in reality. Will Ryuu be able to figure out what's really going on here? [tethysdust]