I'll Be Damned — Kate Hill

Sinister is a demon who has never lived up to his name. He's mischievous at his absolute worst, but that's not good enough for his superiors. They send him to Earth on a mission -- to wreak havoc. Sinsiter knows this is the end of the line for him. Unless he shapes up, he'll be tortured and cast into oblivion. But when he finds a beautiful private investigator in trouble, instead of making the situation worse, he helps.

Jayden is used to working on her own, but she ends up with a rather strange guardian angel, one she can't help loving.

Sinister knows that a demon impersonating an angel must be breaking some cosmic rule, but that's the least of his worries. By loving Jayden and surrendering to his good side, he has sealed his fate and will be destroyed by his hellish masters. Can they find a way to save a demon with an angelic soul?