User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-29963364-20180331030714

I think I know how Ddraig was able to manifest to fight alongside Issei.

Remember that there's one ability Issei hasn't ever used while in DxD mode (true or pseudo).

Dividing Wyvern Fairy. The ability to create multiple small dragons with Ddraig or Albion's abilities (basically making "mini-Ddraigs).

Since the DxD armor is "organic", it would make sense for the Wyverns to be "organic" as well.

So my idea is that Issei, while in DxD mode, creates one gigantic Wyvern, and Ddraig temporarily transfers his spirit into that Wyvern, effectively "reviving" him.