User blog comment:Mnzombie/Issei's Peerage/@comment-115.133.206.167-20130202071925

[The question I received the second most.

“Will Kunou and other girls that have appeared become Ise’s servant?”

Those girls who are not allowed to become a devil due to their position probably won’t. Kunou is the princess of the youkai faction in the Kyoto. There is no way that she can become a devil. But I think they can become his girlfriends even without becoming his servants, you know? There are things like local wives, after all.]

(Quote from Volume 13 Afterword)

So the authors already give the answer to whether some these girls can be Issei's servant.