User blog comment:B214/Season 3 confirmed!!!!!!/@comment-24.104.16.162-20150329163637/@comment-26209278-20150401114007

I think Millicas is named Heir after Rias, due to her not having any children of her own yet. In the circumstance that she doesnt have any children (pfft), Millicas will become the next head (if he also doesn't get offered the chance to become a Maou)