Unceasing Vengeance by J. M. Brister

Gray Tower, Incorporated: A private military company run by billionaire Miles Bryant. Their gray ops missions are executed by the best of the best, including ex-military, special forces, and the exceptionally talented.
Sophie Clark placed her life on hold to help with her father’s presidential campaign. When she is kidnapped and held prisoner by brutal men, she almost loses hope—until she is rescued by an undercover Gray Tower agent. He hates her because she ruined his operation, but she’s stuck with him as her bodyguard as long as her life is in danger.
Gabriel Marshall spent a year infiltrating the group responsible for his sister’s murder, but he had to blow his cover to rescue a senator’s daughter. To make matters worse, he’s ordered to be her bodyguard—a special request by the senator. Sure, he’s attracted to her, but after a job left unfinished, his anger may just ruin his chances of protecting his newest assignment.