User blog comment:B214/Question and Answers/@comment-119.160.168.204-20130309123658/@comment-16281343-20130823062131

The author was able to create those many settings because of Highschool Dxd being a LN with the MC having a harem.

With that he was able to create settings for each harem-member.

As of now after the "Raiser" arc, Asia, Akeno, Koneko (maybe Ravel? vol 14)have their own volumes that tells their own story, complications and tell the viewers how they fell in love with Ise.