User blog comment:B214/Question and Answers 6/@comment-25072311-20190401190525/@comment-29963364-20190425081410

@Demonicjester01

Issei: DxD-G -- The result of combining Ddraig's power with the power of Ophis (because she used her power to "craft" a piece of Great Red's flesh into a body for Issei and bind his soul to it).

Vali: DxD-L -- The result of combining Albion's power with Vali's "Lucifer Power" (made possible by Ophis blessing them).

So, while Ophis is more involved in Issei's case, she served as the catalyst for both forms.