User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-1359322-20130819122035/@comment-1359322-20130819142614

Well the possibles upgrade for Issei could be a Sub-species Balance Breaker, Ryuuteimaru, the Dividing Gear or maybe using the breast of some other girls to power up. Which do you think is more plausible.