User blog comment:B214/Volume 23 discussion blog/@comment-31137559-20170413104227/@comment-31137559-20170413121842

Those with dragon sacred gears have a dragon aura. That aura comes from their sacred gear. Although they aren't physically dragons, they have the aura of one which is why they are weak to dragon slayers. Issei, Vali, Saji, and Siegfried are examples of this. Their sacred gears are the only thing that gives them the aura of a dragon. I'm talking about Issei before he had his body remade. I know that Xenovia and Kuroka wanted to have a child with Issei because of the Heavenly Dragon inside his Boosted Gear at first, but did they know exactly how that work? Or was it just a mistake that Ishibumi made? Things that are said in the beginning of series don't exactly hold true much later in the series.