User blog comment:B214/High School DxD EX/@comment-104.230.243.127-20151127145430/@comment-3435913-20151127154519

Kurenai is a devil with fallen angel and draconian abilities.

Regardless of the fact that Akeno was half fallen, and Issei was a humanoid dragon (formally human), both were reincarnated as devils and as such are still considered devils.

Tanin is no longer considered a Dragon King because he was reincarnated as a devil, so he's no longer considered a 'pure' dragon.