User blog comment:B214/Question and Answers 6/@comment-24098614-20170326152005/@comment-30924344-20170326235531

While Ishibumi wouldn't do this, Issei can certainly improve his demonic power and magic cabalitiy in the future. This is not against Issei's character at all. It's improvement and would means he learns to not to focus on his dragon power/pawn power. It's the same with Sairarog. Both can certainly hsve the potential to elevate their demonic powers to great heights on par or just below Vali's. It's just Ishibumi won't don't that.