Buck, Naked - Chronicles of the Malcolm, #3

To save him, she must strip him bare. Security expert Buck's senses are inhumanly keen, to the point he can see through time and space, and that's just the start of his odd abilities-thank you, alien captors. He also experiences paranoia, flashbacks, and the occasional hallucination-thank you, PTSD. No wonder he fears he could hurt one of the Malcolm crewmates he lives to protect, the closest thing to family he has left after the destruction of his home planet.
Brilliant scientist Aleema can teach him how to use his modifications without triggering PTSD. She's survived the same meddling aliens and suffered even more dramatic alterations. At least her unwanted immortality means she's had time to figure out how the modifications work and use the knowledge to aid other survivors. Helping Buck means spending some serious time together. Time enough to fall in love.
Just as happiness seems within their grasp, Aleema realizes one of Buck's modifications dooms him to an unthinkable future. To save him, she must place herself in the hands of their common enemy and leave Buck to face his worst fears without her.