Starbound

Kira of Eshara has spent twenty-three years being the one her family loves but doesn't need. Not the heir. Not the prodigy. The middle child, steady and quiet and forgettable. So when her planet needs a treaty bride for the most feared warlord in contested space, she's the one who volunteers. She is the one they can afford to lose.
Morak is prepared for a political match. A name on a treaty. Someone who would fulfill the terms and nothing more. But when Kira steps onto his ship, something in his careful plans comes undone, and the bond that takes hold between them is as old as his species and just as permanent.
Because when Morak looks at her, he doesn't see someone replaceable. He sees someone he would give up everything to keep.