Karis by Grace Brennan

Arya Nikolai is finally getting the chance to live her dreams. The rules for her tribe were suffocating, but now the archaic chains holding her back have been broken. Well, most of them. The only thing left for her to do is become a famed War Cat warrior, but it’s forbidden. So she’ll take any chance she can get to show off her skills and break the final link holding her back. And then her inner tiger chooses her mate. It’s someone she’s known all her life. Someone who once ran away from her. Someone who’s still running. Someone who she won’t let slip away from her again.
Karis Sanderson is the head warrior. Tough, strong, badass… and completely terrified of his attraction to the much younger war cat who is dead-set on breaking all the rules. She’s persistent, irresistible, and her father will surely kill him if he finds out. Their relationship must stay a secret, but when someone figures them out, Arya and Karis have more than an angry father to worry about. Now their lives are truly in danger and, secret romance aside, Karis will do anything—even lay down his own life—to protect her.
It’s a secret he never wanted to let out. But to save his mate he’ll have to come to terms with the truth, accept her for who she is, and allow himself to love her.