User blog comment:B214/Question & Answers 5/@comment-178.128.187.173-20150913135947/@comment-108.224.166.119-20150915034717

It's been implied in volume 8 (at least my understanding of it)Rias will marry Issei after she's done with college and also add to the fact issei is going up in rank in the devil society rather quickly so in my opinion issei will become a high class devil during his last year of high school or when in college