User blog comment:B214/Question & Answers 5/@comment-206.75.179.140-20160513232746/@comment-26345952-20160514001500

Issei's DxD mode is Issei using the power of Ophis dormant in his body. But he can only use it because his new body is made of Great Red's flesh, if he was in his human body he would have did at the second he entered the state since it destroys his body greatly.

DxD seems to be his final form but Im not sure, there is still the final arc to go through. Plus de EX side story does show Issei in a more powerfull state (but not in the DxD form).