User blog comment:B214/Volume 22 spoilers/@comment-50.139.232.230-20161022060406/@comment-10330822-20161103002957

I agree with you, but I feel the stuff he's done for Akeno and Koneko is almost equal to what Rias has done for them. Rias essentially saved Akeno and Koneko and gave them a home, but they still hated themselves and were ashamed of whom they are. So, all Rias really did was stop them from being killed. Issei is the one who made them appreciate their powers for what and who they are.

You could almost say that Issei has done more for them getting stronger than Rias has.