User blog comment:Stbk22/Possible power up/Techniques 2 for New Arc/@comment-29362784-20160805000516/@comment-9595671-20160805043522

It's still unlikely Great Red will get directly involved to lend Issei strength, it's the same reason why Ophis can't actually do any fighthing, she's way stronger than most of the cast is and could solve most of their problems. That's why NIdhoggr had to use hostages against her.

Great Red on the other hand? Him getting out of the Gap would end the series instantly due to his strength, I bet if he got out while Trihexa was still sealed away, he could have destroyed the beast while it couldn't fight back which would negate almost all of Volume 21.