User blog comment:B214/Volume 1 - 15 Discussion/@comment-50.172.213.125-20150109174032/@comment-24468758-20150109204735

Rias knew that the FA investigated Issei, but she did not know whether he had a SG. To be able to save him he got the leaflet and after he used it while dying Rias confirmed that he had a SG as the FA wouldn't kill him otherwise. Because of the SG she became interested in making him her servant and after realising that he was worth all her 8 pieces of pawn she couldn't help but make him her servant.