User blog comment:B214/High School DxD EX/@comment-213.120.234.106-20151104110036/@comment-77.48.79.4-20151105180816

Already in first chapter was mentioned that Issei in furure took name Issei Gremory- Hyoudou and Asia took name Asia Hyoudou. Is not stated why. But is obvious that Issei at least married Rias. About Asia, either she polygamously married him as well or she was simply adopted into Hyoudou family by Issei's parents. Personally I think that second option is more probable, as would explain why is Asia only one.