User blog comment:B214/Volume 23 discussion blog/@comment-172.56.17.58-20170417051436/@comment-1359322-20170418010149

I lost count of how many times i explained this. NO Rias is NOT the proper in-line heiress!!!


 * The proper heir-line is Zeoticus > Sirzechs > Millicas.

Devils have low birth rate. Sirzechs and Grayfia has been married for few centuries and didn't give to a child that would inherit the clan. So Rias who was born first, was handed that spot out of necessity since they don't know when Sirzechs' child will be born. However, Millicas was born few years later, so he was place back in-line for heir.

In short, Rias is never the rightful heir, she was chosen out of need because the awaited heir from Sirzechs wasn't born then. Of course, that may not be all the reason why Rias was chosen. Zeoticus probably wanted to use Rias to


 * 1) form familial ties with the Phenex clan
 * 2) expand the Gremory heir-lines. The Phenex clan has 4 siblings. Zeoticus probably wanted to make use of the Phenex's "liquid" to try and increase the number of Pure-blood children in his clan to ensure the clan's survival.