User blog comment:B214/Volume 23 discussion blog/@comment-77.188.210.226-20170509230841/@comment-29963364-20170509231410

After Ophis and Great Red revived Issei (with GR's flesh and Ophis' power), he became a "Human-Shaped" Dragon.

Then Rias re-Reincarnated Issei into a Devil.

So Issei is basically the same as Tannin. A Devil with the physical abilities & characteristics of a Dragon.