User blog comment:B214/Question and Answers/@comment-94.128.225.24-20130127045105/@comment-33447191-20130127050114

Right here:

"Ravel is the official concubine of Issei and is secretly his second wife and this was told in AnimeSukiForum.com it was the site that gave away hints of what happened to Issei and the arrangement with Ravel is similar to Rias with Raiser because her parents want a pureblooded Devil and Issei will grant them very powerful children because now he isn't human at all because he was deemed a True Dragon by Great Red but he is a hybrid Dragon/Devil because of the Evil Pieces inside of him. The Phenex Family made a secret deal with the Gremory Family about the arranged marriage because they now how much Issei hates arranged marriages because Issei believes that a person should marry somebody out of love so he is definitely against this. The parents of both sides will keep him in the dark so Issei gets to know Ravel better and fall in love with her and then tell him but at this point it won't make a difference at all because of how he feels about her at the time so officially she is his concubine with a chance to be a wife at a later time when his feelings towards her become stronger. Ravel in Volume 13 is already trying to change his feelings towards her by asking to come to his house all alone and Issei agreed as dense as he is but was worried of how Rias would take this because Rias would get very jealous about this but Rias already knows about this agreement between her parents and the Phenex Family and won't say a thing about this. Issei is so stupid and dense about this because Ravel's mother told to take care of her but she was giving him hints in a subtle way to get to know her better and start a relationship but he doesn't get the message. The difference in this Arranged Marriage in the one that Rias was set up with Raiser was that Ravel actually loves Issei so she definitely wants this to happen."


 * I came across it when I was browsing through some old forums. I actually don't remember reading it anywhere else, so it could be true (spoilers for Volume 14) or it could be fake (fanfiction).