After Issei and the rest of Rias' Peerage caught his girlfriend Xenovia cheating on him due to Issei moving things slow, it left him heartbroken being used by a woman for a second time and the rest of the Peerage all being mad at Xenovia for breaking Issei's heart, although Rias was beyond pissed off and forbid Xenovia from being involved in any activities with the ORC and told her to stay as far away from Issei as possible.
And to make recovery easier for him, Rias offered Issei to stay with her and her family in the underworld which Issei agreed to so he's as far away from Xenovia as possible.
While down there, Issei started contemplating if his horrible luck with girls was karma for him being the possibly biggest pervert in the world and wondered if this would have happened if he was open to everyone about the reason why he was moving things so slow.
Then slowly but surely thanks to Rias, Issei was able to move on from Xenovia and eventually, Issei found himself and Rias in a relationship.
When Issei came back to school, everyone was happy to hear Issei and Rias were together, all except Xenovia who was starting to be overtaken by hatred and jealousy.
