Honestly I'm not so much sure about Zekka be in Issei's Peerage. In the Harem perhaps yes but I doubt she would turn into a Devil.
I have the sensation that like Cao Cao, Tobio, Arthur and Strada she have to look cool cause she is a very strong human.
However this thread is more about the potential additions to the Peerages of Rias and others, if we want speculated about Zekka in the Issei's Harem and Peerage we should do in threads of Issei's Harem and Peerage:
https://forums.animesuki.com/showthr...25225&page=355
https://forums.animesuki.com/showthr...152040&page=56