Mine to Keep by Tracy St. John

He was the most hated cyborg on Earth, and love is far from his original design. But when old enemies hunt the woman he’s sworn to protect, he discovers a passion for vengeance…and her.

On Earth, he was known as the Warden, and his punishment of prisoners was legendary. Reprogrammed by the president of CyberServe as an assistant in the office…and as her chief researcher of intimate programs…PSM-426021, better known as Alek, is a flawless example of what buyers want in their repurposed cyborgs.
CyberServe’s president Lillian Kwolek enjoys Alek on her terms: no love, no strings attached. It’s easy to keep him at a distance and guard her heart from hurt until an extremist group determined to right old wrongs targets her. Alek is loaded with a security program to keep Lillian safe, and it opens the cyborg to learning emotions.
Alek doesn’t like these feelings, which include a confusing mix of devotion to and wariness of Lillian. She doesn’t like his developing emotions either. The new Alek, who fulfills desires and her need for safety from increasing threats, is too intoxicating a temptation for a woman who can’t trust men, whether they’re flesh or machine.

They can’t resist each other, but past hurts threaten to destroy what they could have…and open the door to killers bent on annihilating cyborgs and Lillian.