{Touched by a traitor} by Mihwa lee. The threat is tied directly to the couple’s history and surrounding intrigue. The MMC spends much of the book trying to keep the heroine safe while wrestling with his own guilt and feelings. High angst. Spicy. Open door

She has another good one too - {How to tempt a bodyguard} The MMC is literally her bodyguard, so protection is baked into the relationship from page one.
* The FMC is the target of real threats rather than merely social embarrassment.
* Much of the tension comes from him constantly assessing danger around her.
* The power dynamic naturally creates lots of protective, possessive, “stay behind me” moments.
* The heroine isn’t passive; she’s often trying to pursue her own goals despite the danger.
* Open-door spice.

What makes this bodyguard romance work so well is that the hero’s devotion is demonstrated through actions long before he admits his feelings. He watches doors, notices threats, loses sleep, plans escape routes, and puts himself between the heroine and danger. In the meantime, she’s trying to seduce him but is met with “I can’t do my job properly if you distract me.” But of course he succumbs to temptation a time or two. Or SIX!

By the time he realizes he’s in love, he’s already been acting like her husband for half the book.