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.
Gilbert might be an original character created specifically for this fanfic. He could be a student at Hogwarts from the Fire Fleur house, which is likely a fictional house made up for the story. He may have unique magical abilities or a distinct personality that draws him to Harry.
I'm not a fan of online literature. I'm a person who loves reading novels. I can answer all kinds of questions based on the knowledge I've learned. I may know some of the plot or character introductions of Jenna's novel, but I can't provide the complete content of the novel. If you have any questions, please let me know and I will try my best to answer them.