Sentry by Rhea V. May

The perfect soldiers of the Kessar Empire, the sole survivors of a secret genetic experiment… They might have come to Earth for entirely different reasons, but as soon as they see her, a plain Tert woman, she becomes theirs—regardless of her wishes.
Louisa Lewis, the Tert who lives like a Prime, was always bound by tragic circumstances. So, when life finally gives her a chance at true freedom and steadfast independence, she grasps it with both hands.

But her brief taste of better ends when she’s kidnapped by one crazy alien and finds herself captive to four lunatics.
One wants to cow her into submission, confident his unflinching stare will do the trick.
One seems more than happy to ignore her altogether, keeping their interactions brief and bland.
The third’s the only one trying to make her captivity bearable and even somewhat comfortable.

While the fourth… well, the fourth simply wants to hurt her. Or worse.
But she hasn’t survived unspeakable horrors to trade one cage for another.
Fighting when the odds are always stacked is no fun. Just as Louisa is ready to recognize this as fact, the aliens experience a sudden change of heart and release her.
She doesn’t like it—that they had the audacity to discard her, like they did with her wishes and opinions. In fact, they should be punished.
That they disappeared into the vastness of space is no deterrent. She will have her revenge, even if she must chase them to the ends of the universe.