User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-9595671-20170417182258

Yeah, Rias is only acting as the stand in due to Sirzechs having become the new Lucifer and being unable to take over, when Millicas comes of age he'll replace her.

I believe it was stated in the EX novel that he has already done this IIRC