User blog comment:B214/Light Novel Volume 16 spoilers/@comment-202.152.199.187-20131201114942/@comment-1359322-20131202102306

We're still not sure how it is actually, it could also be Ddraig once fought against Balor and Balor's power somehow got stuck on Ddraig and into the Boosted Gear too. Then Issei's interaction with Gasper awaken that power.