Unsidhe Assassin by Leona Windwalker

Known as Ghost, Jase is an assassin with but one soft spot- his kid sister. He just wants to earn enough money to live life the way he wants while making sure creepers stay away from her. He certainly doesn’t believe in fairies, not even dark ones.

Lord Willow needs an assassin and it has to a human one. Humans are hard to come by on his side of the Veil, but he’s not above tricking his way into bringing one across. Ghost will be perfect once he’s molded into what Willow needs both in and out of bed. First, though, Willow has to lure him across the Veil with a bargain. Then he has to break him.