Feral Guardian by Ember Holt

A prophecy of ruin binds him to her.
She wants nothing more than to escape.

Lily has never been the delicate flower she was named after. She preferred silly pranks and dirty poetry to needlepoint and politics, which is why the queen sent her off to a “proper” royal academy. Now that her magic is developed and her manners are passable—barely—it’s time for her to return home and attract a powerful man to rule by her side. Except Lily doesn’t want a king or a kingdom. She wants freedom. And nothing feels more freeing than adventuring with her bodyguard, Alastair.
Alastair is an honor-bound knight, tied to a prophecy from his people, and he doesn’t experience desire. At least, that’s what he likes to tell himself. In truth, he wants nothing more than to sweep his ward off her feet and take her away to a dark corner of the world where he can keep her safe from that dark prophecy. There’s just one problem: the prophecy states if Lily doesn’t become queen, the tentative peace in the kingdoms will crumble.
When their journey home from the royal academy lands Lily in the custody of cutthroat pirates, Alastair will stop at nothing to protect his princess. But the biggest danger of all may be Alastair himself, because if Lily doesn’t stop trying to make him break his vow of chastity, he may forget about honor and risk everything to have her to himself.