Fallen Angel. Sainted Sinners, #1

The first rule of paranormal dating: never fall for a fallen angel.  Vesper Emery isn't a soft and cuddly type of girl. A butt-kicking bounty hunter, she spends her nights tracking down demons. and her days alone. When she's forced to take on sinfully handsome fallen angel Kirael as her next assignment, Vesper has no idea just how hot things are about to get. Alpha bad boy Kirael never saw gorgeous, thorny Vesper coming.
mostly because she's trying to tackle him and stab him with her sword. On a journey to find redemption, fiery-hearted Vesper is a temptation that Kirael can't afford. Once they stop trying to kill each other, they realize that they both have the same impossible goal: break into Hell and steal from the devil himself. Between snarky remarks and fighting for their lives, a new and fragile relationship blooms. The second rule of paranormal dating: if you're lucky enough to find a soulmate, you'd better fight like hell to make it last.