User blog comment:Crimson and Darkness/DxD Updates 2~/@comment-30924344-20170826132209/@comment-73.111.74.35-20170827213825

I thought Asia fell in love with him first, since Issei rescued her from Raynare and her group of rogue fallen angels. And, he was her very first friend.

I didn't think that Rias fell in love with him until they were training for the rating game against Riser and Issei said that he liked her for who she was. Him saving her did cement her love for him, but she didn't actually fall in love until then.

She did say herself that Kiba was an honorable Knight so she couldn't ask him to take her virginity, so that was her last option. It only shows that she held a lot of trust in Issei to do that with him.