User blog comment:B214/Shin DxD 1 discussion/@comment-2607:FB90:3851:A5D3:0:14:ECCB:7501-20181029115052/@comment-108.7.145.117-20181029203214

Why is Rias a Wall?

Elementary anon, it's because she is his main love interest, she was his master and she is his bride to be. Not only that but her team is at least 1/2 made up of Issei's friends and comrades.

Now imagine that you and your friends had to fight your wife/girlfriend/etc. and their friends who are also YOUR friends and friends with one another... Issei didn't hesitate against Vidar because he was motivated to battle a rival, a friendly one, but a rival nonetheless. With Rias he has never had this rivalry and he would thus hesitate to fight her. Not only that but she is far more accustomed to being a strategist and King, and has the power (both her own and team's) to back it up, unlike Sona who's technique based style lacked the sheer power required to face off against god level powers.