User blog comment:B214/Shin DxD 1 discussion/@comment-34061653-20181119044210/@comment-82.52.112.59-20181119215802

Rias' mother speaking to her told her that Sirzechs didn't have a harem because he was a Maou, but since Isse didn't aim for that role she had to fulfill her role as legal wife and handle the harem.

Anyway it seems that Issei's position won't be exactly as Maou but a sort of wild card, a jolly.,