User blog comment:B214/Volume 22 spoilers/@comment-213.120.234.106-20160720102919/@comment-24.113.252.213-20160721051943

I have a thought:

Bina Lessthan (Issei's Queen in the IRG) could be the mother of one of Issei's 20+ kids. She might be from a family of low/mid-class devils, and became inspired by Issei who is a Reincarnated Devil, yet became the strongest young Devil, a hero to the Underworld, and a High-Class Devil, all in less than a year!

So she does pretty much what Ravel does, she grows to respect and admire Issei, and wants to become a strong and famous devil too. In order to do this, she decides that joining Issei's peerage as his Queen would be her best option. But she needs to see if Issei has potential to be a good King, so she tests him in IRG. Issei proves he's got potential, and she decides to join his peerage officially. They spend time together, she evenually falls for him, and they have a child.

This is half my theory of her identity, and half my theory about Issei's wives & kids.