User blog comment:B214/Volume 21 info & spoilers/@comment-95.90.227.186-20160507145425/@comment-1359322-20160508014332

"So Tannin and Issei aren't Devils or Devil-Dragon Hybrids. They are Dragons with Devil Powers and Characteristics."

It's the opposite, Ise and Tannin are Reincarnated Devils with Dragon powers. It's same with others. Koneko is a Reincarnated Devil with Nekoshou powers and so on. The Evil Pieces are created to help replenish the number of Devils, why would they grant other species Devil powers and get nothing in return. So they become Devil as a race/species the moment they got reincarnated but retain the trait they originally possess.