In this manga there are two siblings, the boy can make weapons from his body and get new weapons by eating monsters and the girl has psychic powers. I read it in 2008. In chapter one (the only chapter I ever read), the two of them lose their parents by attacked in their own house by a monster (I don't remember what the monster is called). They see the whole process, from the killing to the monster eating both of their parents, then when they about to be attacked, their power suddenly appears, they kill the monster and eat it. From then on they start killing monsters and eating them. They grow stronger by eating the monster and that's it, chapter one ended there. I think the time settings is in the present, Japan. The genre, if I'm not wrong is action sci-fi. It's a Japanese-style black and white manga.