User blog comment:B214/Volume 23 discussion blog/@comment-185.112.105.215-20170418110602/@comment-31137559-20170418195252

Well it doesn't have to be stated. Why do you think that Issei's countdown time for his Balance Breaker at the beginning of the series decreased? How do you think Issei has been able to Boost so many times. How do you think he beat Barakiel after he used most of his stamina when he went DxD mode? Issei constantly trains and is building up his stamina bit by bit.