User blog comment:B214/Talk About What You Want! 2/@comment-108.27.243.235-20140707032420/@comment-1359322-20140707103010

You can't really say that Rias' doesn't know about it. Knowing Rias, she would've done some research on Riser and his servants, remember when Riser kissed his servants, Rias wasn't even surprised because she probably knew about it.