The black cat is surely one of the main characters. It could be a curious, adventurous cat with unique abilities. And then, there's Rinslet. Rinslet might be a person, or perhaps a magical being, depending on the nature of the fanfiction. There could also be the train conductor or other passengers on the train who play important roles, like helping or hindering the cat's journey.
The main characters would be the black cat, which could be a wise and mysterious feline. Rinslet, which might be a beautiful and kind - hearted character. And the train itself could almost be a character. It has its own rhythm and route, and it affects the journey of the cat. There could also be a little girl or boy on the train who befriends the cat and helps it in its quest related to Rinslet.
Obviously, the black cat is a key character. It may be a mischievous cat that gets into all sorts of trouble on the train. Rinslet is another main character. It could be a powerful entity that the cat is either trying to reach or protect from. Additionally, there might be a villain on the train, someone who wants to stop the cat from getting to Rinslet or causing harm to both of them.
In this fanfiction, the black cat could be the guardian of the train. One day, a strange force starts affecting the train's normal operation. The cat senses that this force is related to Rinslet. So, it decides to leave the train and venture into the world of Rinslet. There, it finds that an ancient curse is causing problems. The cat then uses its intelligence and some unique skills it has (like being able to move stealthily and having sharp senses) to break the curse and save both the train and Rinslet.
Naruto himself is likely to be one of the main characters. Since it's a Naruto fanfiction, he would probably play a central role. Maybe he'll be the one who discovers the power of Black Lightning and tries to incorporate it into his own fighting style. Another likely character could be Sasuke. He's always been Naruto's rival, and if Naruto starts using new powers, Sasuke would surely be interested in it too.
Obviously, Harry as the reborn Black would be a main character. Sirius Black would probably also play a major role, as he's an important part of the Black family. And Bellatrix Lestrange could be another key character, her relationship with the Black family and her own dark nature would create a lot of interesting interactions with Harry in his new identity.
For the black cat character, you can start by giving it unique mannerisms. For example, it always arches its back when it senses danger or it has a habit of rubbing against Eve's legs when it's feeling affectionate. In terms of its backstory, it could have been a stray cat that was adopted by the train's conductor in the past, which gives it a special connection to the train.
Serafina is surely the central character. Alongside her, there could be a mentor - like figure who has some knowledge about the black cloak. This person could guide Serafina in understanding the true nature of the cloak. And then there could be some minor characters in the fanfiction, like the villagers or servants in the setting where the story takes place, who add more flavor and background to the overall narrative.
Obviously, Harry would be one of the main characters. Given the 'Black' in the title, characters from the Black family like Sirius Black or Bellatrix Lestrange could be important. And of course, Rumpelstiltskin would be a central figure as well. He would be the one causing all the trouble related to the inheritance.
Obviously Naruto would be one of the main characters. And since it's about him being the son of Black Canary, Black Canary herself would be important. There could also be characters from the Naruto series like Sasuke and Sakura. They might be curious about Naruto's new powers and his strange origin.
Harry, of course, is central. Sirius is likely to be like a father figure to him. Bellatrix Lestrange could be an interesting character in this scenario. Although she's a bit of a dark character, her presence in the Black family and her strong beliefs could create a lot of drama and conflict. Also, Kreacher, the house - elf of the Black family, would be around Harry a lot, and their relationship could develop in unique ways compared to the original story.
Sasuke could also play a major role. His rivalry with Naruto might take on a new dimension with Naruto having the power of the Black Dragon Emperor. He could either be jealous and try to compete even more fiercely, or he might be intrigued and eventually team up with Naruto in a new way.
The main character is the narrator. And of course, the black cat is also a very important character. The narrator's relationship with the cat drives the whole story.