User blog comment:Piyush Banerjee/Shin DxD 2 discussion/@comment-36663391-20190118224842/@comment-10330822-20190122063534

It also might be a nice way around his "Worthless" ability, because it weakens as Ingvild "strengthens". And since Diehauser hasn't had time to analyze Ingvild, because she's never competed, that would mean he couldn't use Worthless on Ingvild either.

Still, I feel she's actually only meant to be help with the inevitable Indra fight. Issei may be beating Diehauser another way.