In this ova of Owari no Seraph (a wonderful work of art that is still without a sequel) we can see the story of Shahar, a vampire who was human and who missed his little sister Lana. That is why he kidnaps a human and makes her like his sister.
It's a parallel story, but this way we can know that in Owari no Seraph there are more vampires than were previously humans and that many of them don't want to be that "monster".
