User blog comment:B214/High School DxD EX/@comment-68.53.129.54-20170413035039/@comment-31137559-20170415001443

Well besides the 'not having a love interest' part, you are spot on. Griselda doesn't have a love interest and I don't see her joining the harem because I see her serving the same role to Issei that Grayfia does: as his older sister. And plus he would have a devil onee-san, an angel onee-san, and a fallen angel-sensei. Quite fitting.