User blog comment:B214/Volume 22 spoilers/@comment-72.50.80.249-20160610225905/@comment-27876048-20160611023543

Right, it's one of the things I love about this series we actually see Issei's personality developing and maturing and also how Issei's perception of the world changes based on what he learn or what situation he finds himself in. Which is different from most series which has the MC with their personality and views already set and there very little character development.