User blog comment:B214/Volume 23 discussion blog/@comment-31764731-20170421191906/@comment-31137559-20170421234138

The only nobles are those of the 72 Pillars. Now Issei will be marrying into the House of Gremory becoming a noble through being married to Rias. Reincarnated devils that rise up in rank enough will have status equal to nobles but won't actually become a noble unless they marry into a noble family like Issei will. Now you can have a family and your name can be known across the Underworld, but that doesn't mean that you're going to create a noble household.