User blog comment:B214/Question and Answers 2/@comment-79.56.162.181-20140204084111/@comment-1359322-20140204092244

Well currently there are theories that if Cao Cao joins DxD, Georg, Leonardo, Jeanne and Heracles might do the same but that remains to be seen. Plus we still don't know if Cao Cao will interact with Issei at all for now. He doesn't seem to be appearing in Volume 17.