Supernatural, Action, Ecchi, Shounen, Sci-Fi

It all started out with Jain, an old man. Not many people really know his past except for, well, a mafia boss,and a few others. At the beginning of our story, Jain finds a girl, bloody and knocked out on the sidewalk, so, unlike most people, he decided to shelter her in his home. But what is she, and does Jain know what he's doing by feeding, loving, and caring for her?