User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-1359322-20141207104448/@comment-24502975-20141219001048

I disagree. We do know about Kuroka's history, but there's certainly a lot more to be covered there. The Vali Team was pardoned for their Khaos Brigade crimes, but Kuroka is still a wanted Stray Devil. If the higher-ups found out that [DxD] has been harboring a fugitive to the Underworld, major problems could arise for the Maous, not even mentioning how this affects Kuroka and how it could even be resolved. We also know literally nothing about Le Fay's past except that she was a part of Golden Dawn and followed her brother when he joined the KD; I would love to get more info on her backstory. Kunou I agree there's not much history for her, thus that is the logical Youkai volume.

More likely it'll be Rias -> Asia -> Akeno -> Koneko & Kuroka -> Le Fay & Kunou.