Adrik by Susan Bliler

Ember and her sisters have known nothing but captivity, traded like commodities from one cruel captor to the next. But when a group of warriors known as the Titans rescue them, they’re forced to reckon with a new kind of threat—one that comes in the form of the brooding, dangerous leader, Adrik. Despite the Titans’ rough exterior, Ember can’t deny their surprising decency or her growing attraction to the Commander. But she can’t afford to let her guard down. Not while her sisters’ safety is at stake.
Adrik is no ordinary Titan. As a rare shadow Titan, he’s dedicated his life to the service of his people, bound by duty and devoid of personal desires. Until he encounters Ember. Among the rescued females, one stands out: Ember’s resemblance to a shadow female—a near-mythical rarity in his species. Shadow Titans are only allowed to mate with shadow females, and the dwindling number of such women has left Adrik with little hope for a future. Until now.
Caught between the ghosts of their past and the uncertain promise of a future with the Titans, Ember and her sisters must decide whether this unexpected refuge is worth the risk. But when another faction of ruthless galactic warriors offers to purchase the females, Adrik is faced with an impossible choice. One that leaves Ember wondering if he will sacrifice everything to keep her or if his duty will demand he abandon her.

*This is book 1 in the Titans series. Each book focuses on a different couple and can be read as a stand-alone.