when Tsukasa was younger, he was given up for adoption by his parents, where he met many wonderful people. but there was one thing he stuck with best, Rui. All the adults told them they couldn't go outside because of the virus but in reality it was because of the vampires that one day they crept into adoption and killed most of the children. Tsukasa had to watch his Rui die in front of him. the boy managed to escape from there.