One great place to find rebecca and nathaniel fanfiction is Archive of Our Own (AO3). There are many stories there with different themes. Some might focus on their relationship development, like how they first met and how their feelings grew over time. Another option could be Fanfiction.net, though the selection might be a bit more limited compared to AO3.
There are fanfictions that delve into their backstories. For example, there might be one that explains Nathaniel's past relationships and how that affected his connection with Rebecca. By exploring different fanfiction platforms and using specific search terms like'rebecca and nathaniel romance' or'rebecca and nathaniel angst', you can find a wide variety of fanfictions.