Alien Mercenary's Destiny - Lathar Mercenaries: Warborne, #4

He saved her from certain death. She didn't count on him being a. nerd. A Tarviisan pit fighter forced to fight for survival her entire life, Zadaenae is sure she's about to breathe her last when she's cornered during a breakout. But help comes from an unexpected quarter; an alien with the infamous Warborne Mercenary team. Only her savior isn't an experienced mercenary, he's a scientist without a clue about fighting. Wracked with guilt over his injuries saving her, she takes a hire with the Warborne to stick around, even though the last thing she needs is the scarily intelligent, utterly fascinating human finding out her deepest, darkest secret. He nearly died.
Then he became something else. Something. not human. Dr. Eric Archer has all the smarts he's ever needed, but multiple degrees don't mean anything on the alien mercenary vessel, Sprite. Instead of working in a lab, he's had a crash course in battlefield tactics, field stripping alien rifles, and shooting just to stay alive and earn his place among the lethal aliens. She's an alien. And a princess.
He needs to up his game to have any chance. When rescuing a beautiful damsel in distress almost kills him, an experimental alien treatment saves his life. And makes him more than human. Makes him something. else. Forced to deal with more new muscles than he knows what to do with, plus hormones and instincts no human has ever encountered before, Eric must navigate his new abilities and prove to alien princess Zadaenae he's the only man she'll ever need. But he's not the only one with eyes on his beautiful alien princess.
She's someone else's means to an end, and this scientist must use everything at his disposal: new skills and old alike to try and save the woman he loves.