User blog comment:B214/Question and Answers 4/@comment-59.97.96.92-20141127195338/@comment-16281343-20141127195814

That's Bikou disguised as Rias. Rias is even holding a banana. The Vali team went to the Underworld and cosplayed as Oppai Dragon as disguise.

If you look at the other illustrations, Vali's armor is Issei's and Kuroka cosplays as Hellcat-chan.