Blue Exorcist, Black Wings
Rin and Yukio Okomeru were believed to be the only direct children of a demon and a human left on Assea. This isn't quite correct. There is another, one who's powers are unique. This half-human grew up in Gahena, unlike the twins, and is the child of the Eyes of Satan, the bird king of the skies. However this child has been missing for several centuries. So what happens when the the twins meet this mysterious person who's secrets are darker than even their own.