Carsyn by Grace Brennan

Carsyn Katz lost half of her world when she was fourteen years old. Now, she fights as a Rogue Enforcer with the other half, preventing tragedies from happening to other children. But when a new mission separates her from her twin, it hits a little too close to home. She’s fighting with someone she doesn’t know, against an enemy she can’t get a handle on, while kids continue to disappear—on her watch—and she can’t shake the feeling that maybe this time, she’s in over her head.
Kingston Powell is used to living his nomadic life alone, content to focus on his missions for the Rogues. But when his boss pairs him up with a gorgeous spitfire who crashes into his life with the force of a wrecking ball, he’s instantly smitten. It’s easy to see that something haunts her, and as the mission goes on, his concern for her grows. How can someone so strong be so fragile at the same time?
As they race to save the children from the evil stalking them, Kingston is racing to save his mate, as well. Will they rescue the children and give everyone a happy ending? Or are they destined to crash and burn in a fiery inferno?