User blog comment:B214/Volume 17 spoilers./@comment-24098614-20140318165611/@comment-25051741-20140612060111

The reaction that surprised me was Akeno's. I remember early on Akeno telling Issie she wanted to have an affair with him (That's in the anime 2nd season... I haven't read the complete LN yet... but I'm luvin it!). She seems more interested in being his Mistress than his wife. My understanding of Japanese culture is that  "Mistress" (Japanese equivalent?) is a position of honor and she is considered to be outside of the ranking system for wives/concubines. Not inferior to any wives except in matters of inheritance.

I felt like Akeno had the position of Mistress sown-up. So Issei adding another member to the Harem should not be a shock or threat to her. She always semed to show more confidence in their relationship than hurriedly leaving at the thought of a new non-rival.