User blog comment:B214/Volume 23 discussion blog/@comment-77.188.210.226-20170509230841/@comment-27083298-20170510175736

Issei is still being called a Heavenly Dragon. Dragon Gods belongs only GR, Ophis, and Lilith.

DxD is Diabolos Dragon God is called like that because he is using GR and Ophis's power.