User blog comment:B214/Volume 17 spoilers./@comment-24098614-20140318165611/@comment-1359322-20140612115311

Hmm, personally i feel that Ishibumi is starting to get serious about pushing Issei and Akeno's relationship, and it's not a mistress or the one Akeno said like in Volume 4 or for fun like usual, a more serious one.

In Volume 14, there's that declaration by Issei. In Volume 15, Akeno's dream as as child. And in Volume 17, well it's already stated above.