The Idea Factory - TWITSEC and the Mysterious Prisoner
These are two separate book ideas, but they're kind of related. With TWITSEC, it's using time travel for witness protection. It would either be third person following the family after arriving in the past, with them adjusting to the new time period, or possibly first person with the love interest finding out about them.
With The Mysterious Prisoner, it's focused on the guard that is placed on her cell. The two start to talking and they fall in love. The guard wonders why she's locked up, eventually finding out that she is a time traveler from the future, trapped in the past. They escape together maybe, only for her to turn on him so that she can accomplish the mission she was sent back there to do.