User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-10330822-20170417193043

True, Rias is the legitmate heir, and unless she gives up the title, it is never going to Millicas. EX hints that she does forgo the title and gives it to Millicas. Part of me suspects its because Issei gets a title that is even better, and she just decides to be his consort instead of a regent herself.