User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-34061653-20190122110131/@comment-34061653-20190127050622

You are right that Rias started off stronger than Issei even though he quickly surpassed her by volume 5 and has remained stronger ever since. In fact the gap only increased and by volume 20, the gap was so wide that Rias wasn't even on Issei's radar. But in her defense she did catch up somewhat with her Aeon Balor Princess form. But still, as the main wife of Issei, you would think she would be the strongest in his harem but sadly that's not the case. On her own, I can see Akeno, Xenovia and maybe Koneko (3 tailed state) stronger than her. Even with her Aeon Balor Princess, you can argue that Asia is equal to or superior when you count her OP BxB with Fafnir.