User blog comment:B214/Volume 22 spoilers/@comment-201.215.165.7-20170213165134/@comment-30924344-20170215092751

@B214 Issei will most certainly become an ultimate-class devil. So pretty much Ex, Kurenai, Zen, Airi, Helmwidge and Robertina will be considered high-class devils by birth, along with the fact that the girls will be high-class devils themselves. Children born to an ultimate-class father and high-class mothers pretty much guarantees high-class status. I think Issei will already be an ultimate-class devil by the time he has his first child, Airi or Kurenai.