User blog comment:B214/Volume 23 discussion blog/@comment-86.143.86.100-20170203180136/@comment-31137559-20170203190724

Using his demonic power to improve will only make him a knockoff of Vali. I know a lot of devils do it, but Issei doing it this late in the series when he's already so powerful would be an asspull. If you are talking about mentally, I don't think demonic powers do that much. Well what do I know? This is something that Issei needs to develop on his own. He just needs to grow into the role and get more experience. Remember that he has only been a devil and has known of his Sacred Gear and the supernatural world for only a year. I know that he has gotten much stronger and that he has fought many powerful opponents and have met many important figures, but it still has been only a year. With all these things coming down on Issei one after the other, it is kind of natural for him to fall behind a bit.