Revenant is Reaver's twin brother but he was raised in hell and without his memories(because what is done to one twin must be done to the other) he believed he was a fallen angel for 5000 yrs. Now that he has his memories back he's pissed that his brother was raised in a safe and loving environment while he was tortured everyday of his life.
2 weeks after getting his memory back he meets Blaspheme and decides he needs to tap that but she tells him no, not something that's ever happened to him in his very long life. Blaspheme works at UG and thinks that the hot tool is still a tool and she needs to protect herself as her enchanted disguise is wearing off. Blaspheme is the child of a fallen angel and an angel, a creature that is always hunted down and killed.
This was an angsty relationship on several levels and a good way to tie up a lot of loose ends in the series. Only a couple of books left now :)