User blog comment:Xhatefate/DxD Updates~/@comment-29362784-20161006021220/@comment-9595671-20161007231440

I think the main reason Great Red tends to not be involved in the story often is cause he'd pretty much solve many problems with her power, the same out-of universe explanation for Ophis not fighting.

I do worry how GR would show, the best chance he had of showing could have been when Trihexa was freed. If they had somehow managed to move Trihexa in there they could have fought without the same dangers, but since no one knows how GR got in... that likely wouldn't have worked.

Plus we don't know if Trihexa could have gotten out, for all we know Great Red could have gone in the gap of his own will. Even if neither could it's said he AND Ophis were the only way to destroy Trihexa for good, and with her depowered even Shiva and Lilith aiding her? Probably not enough.