User blog comment:B214/Question and Answers/@comment-94.128.225.24-20130127045105/@comment-5882552-20130127053301

"... so officially she is his concubine with a chance to be a wife at a later time when his feelings towards her become stronger"

By wife, you mean second wife, right? Because I would be confused why the Gremory would allow Ravel to become Issei's legal wife instead of Rias'.