User blog comment:B214/Question and Answers 6/@comment-24098614-20170326152005/@comment-30924344-20170326155512

Okay I just realised that people have got this entirely wrong. Vali has never been stronger than Issei sinc Volume 4. Their strength and stamina has been on pretty much par with one another throughout the series. Vali just has better demonic power and durability than Issei. That is all. All the claims of incidents where Vali is supossedly has more strength than Issei was really him just being more durable to fight longer than Issei.