User blog comment:B214/Volume 21 info & spoilers/@comment-95.90.227.186-20160507145425/@comment-24.113.252.213-20160508000109

The Evil Pieces don't actually turn you into a real devil. They just give you the characteristics of a devil (Demonic Power, Bat-like wings, enhanced phisical abilities and senses). The only reason Humans can still use Sacred Gears and wield Holy Swords is because they haven't become devils, but have been given devil abilities. And the reason these two opposing qualities can be possessed is because the Original Satans and the Biblical God are dead and can't maintain the balance between the two.

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