Jenna is likely the main character in the story. She could be a young girl, a teenager, or even an adult. Since the story is named after her and the black cat, she's probably the one whose experiences are central to the plot.
As we don't have much information about this specific story, Jenna could be a protagonist who is a slave. She might be a symbol of the resilience of slaves in the face of the cruel institution of slavery.
I don't know what you want to know about Jenna's novel. Please provide more background and context information so that I can better answer your questions.
One common theme is the exploration of their relationship development. Since they are both characters from Fairy Tail, many fanfictions focus on how they get to know each other better and fall in love.
Another theme might be the challenges they face as a couple, perhaps due to their different personalities or the expectations within their fictional world. For example, Elfman's tough - guy exterior and Evergreen's more aloof nature could create some interesting conflicts in a relationship.