User blog comment:B00m666/Highschool DxD - Question about Issei/@comment-1359322-20131204005537/@comment-3125355-20131210171318

the anime ended with vol4 of the novels, the explanation, would need to cover till at least volume 8 of the novels if i remember correctly, or was it 10. so its not going to be very easy. to put it simply, it has got to do with the fact that Akeno is also a half breed, with her dad being one of the heads of the  Grigori.