User blog comment:B214/High School DxD EX/@comment-201.239.133.207-20170104020556/@comment-1359322-20170210022319

Ex is not a Pure-blood Devil. A Pure-blood Devil originates from a Devil origin of both parents. Issei does not originate from a Devil, he originated from a human. So Ex in fact is a half-Devil because one of his parents originated from another species as opposed to Rias who's parents originated both from a pure Devil origin.

That's why a child of two Reincarnated Devil is still a Reincarnated Devil, because they are reincarnated from another species and not from a Devil origin.