I can confirm Rias Gremory does eventually marry Issei Hyoudai. Their relationship evolves from master-servant to genuine lovers over the course of the series. The wedding happens in the later light novels, and it's every bit as extravagant as you'd expect from the Gremory heiress - think flying carriages pulled by dragons, a guest list including Satan himself, and enough magical fireworks to light up the underworld for a week. Their marriage also politically unites two powerful factions, with Issei becoming a High-class Devil in his own right. What makes their union special is how it balances Rias' noble upbringing with Issei's chaotic energy - she keeps him grounded while he helps her break free from stuffy traditions.