User blog comment:B214/High School DxD BorN Episode 10 Review/@comment-9339850-20150607040108/@comment-1359322-20150607041650

Actually that is because Season 3 was made to test both Ise and Rias. The anime merely used Loki to show that. From the anime-point of view only:

For Issei, it's his starting awareness of how much Raynare scarred him, but Rias' existence was a deterrent to that. We learnt that from the JD last episode.

For Rias, it's her feelings for Issei. She is aware that she loves Issei, and can be jealous, but since she cares for the girls (Akeno, Koneko and Asia) too, that jealousy starts to make her have conflicting feelings then the others starts to get closer to Issei, Rias' becomes even more uncertain of herself. Unsure of what to feel, should she be happy, thankful or jealous, angry, frustrated. So the curse end up creating an Ise that she can be free of those painful conflicting feelings.