User blog comment:Mnzombie/Issei's Peerage/@comment-139.168.116.203-20131206051903/@comment-9339850-20131206095932

thats exactly what i was thinking when i saw Rias as the queen; I would assume that they could be, i mean it'd be a weird thing to exclude, but i guess since the Evil Pieces system was created to increase devil numbers, it'd be essentially against the purpose if you use a piece on another devil.

But Ravel is pure-blooded and she was made into a bishop by Riser, so if we consider her case it would seem that it can be done, but on the other hand she didnt have her own peerage (maybe wasnt old enough?) so maybe she was able to fit through some kind of loophole.