Revenge River by Lindsay Cross

Merc:
His name is Merc.
He has no past.
No future.
Only a thirst for revenge against the traitor who betrayed his team and his country – Agent Jack Mankel.
When he’s handed a mission to rescue a sitting US senator’s daughter from the traitor’s clutches, Merc jumps at his chance for vengeance, never dreaming the girl he rescued could be his downfall…

Nightshade:
She walks in the night.
She is a shadow.
An operative so elite even the president doesn’t know of her existence.
Her whole life, she’s lived to serve, raised in the black-ops team her father, Jack Mankel, created. Then her whole world is turned upside down when she learns the sister she once thought dead is really alive; and to save her, Nightshade will have to take on the hardest mission of her life…

Merc knows he doesn’t deserve her. Nightshade knows she can’t have him. But a connection so powerful neither of them can resist draws them closer, even as a betrayal so deep threatens to destroy their very existence.