User blog comment:B214/Shin DxD 1 discussion/@comment-2607:FB90:3851:A5D3:0:14:ECCB:7501-20181029115052/@comment-108.7.145.117-20181030020701

"its a game so it's not serious, she's a princess, so not warrior, he joined because of others. There's more to it than that."

Actually that applies to YOU.

It's not BECAUSE she is a princess she is not a warrior, but because of who she is and what she does she is a princess. She is not on the frontlines most of the time, that is the point of a rating game, as an imitation of chess with the pieces and the Kings strategizing first and fighting personally Second. Issei is seen as uncouth for his direct fighting on the front and

It's not because of it being a game that it isn't serious, (you're wrong on both there). It's all the more serious and all the more problematic. It's not the fight itself so much as what choices it gives that are the challenge for Issei. Your understanding of this is based solely on the idea that Issei is confident in battle... yes, against rivals and enemies, not against his lovers.