To Love a Sentry by Rose Wulf

Rochelle Bailey had been born into a technological world devoid of magic. She had never imagined anything different for herself, only pushed forward, slogging through tragedy after tragedy, waiting for her misery to end. Then she woke up, naked and confused, in an alternate world so full of magic she could practically taste it. Except she still had none.
Until the day she met him. Aric Vardanyan, the Sentry of Yafae … the very grown-up version of an anime protagonist she’d once watched on TV.
Aric volunteered to take Rochelle into his home and teach her how to control her magic. There was something about her that intrigued him and compelled him to take action in a way he wasn’t used to. So he trained her, he made her strong, and he did his best to resist his rapidly rising attraction to her in the process.
But the fragile balance of their relationship tipped forever when he received a secret assignment to investigate a rumor that could lead to war … and his true enemy took the opportunity to put their hands on Rochelle.

He was the kingdom’s most powerful sorcerer, and for her, he would throw it all away.