User blog comment:B00m666/Highschool DxD - Question about Issei/@comment-9339850-20131203201731/@comment-9339850-20131203215357

Yes they are a couple as of Vol 10, and engaged as of Vol 13 (but timewise the events in Vol 13 happen through 7-12 i believe); Rias and her family are the only ones that know about the engagement atm, though Issei has no idea on that matter eventhough he participated in the engagement ceremony