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.
The main character is Serafina. She is a curious and brave girl. Alongside her, there are other characters like the people in the place she lives. Some of them might be victims of the black cloak's mystery, while others might be involved in some way or another. But Serafina is the one who drives the story forward as she tries to solve the mystery of the black cloak.
It could be about Serafina having some sort of special adventure while wearing the black cloak. Maybe she discovers a hidden power within the cloak that helps her save her friends or solve a mystery in the story.
How about a talking cat? The cat could have been a victim of the black cloak's magic but managed to retain some of its form. It could act as a guide for Serafina, using its knowledge of the manor's hidden passages and secrets that it learned before being attacked by the cloak.
The main plot revolves around Serafina who discovers a mysterious black cloak. It's a story full of mystery and adventure as she tries to figure out the secrets behind the cloak and the strange happenings in her world.
Well, a plot idea could be that the black cloak starts to have a mind of its own and tries to control Serafina. She has to fight against its influence and find a way to break the connection. Or, what about a story where Serafina uses the black cloak to enter a parallel dimension where all the people who have been taken by the cloak are trapped. She has to rescue them and find a way to close the portal to that dimension. Also, a plot where she discovers that the black cloak was once a good magic item but was corrupted, and she tries to reverse the corruption.
Obviously, the prince of peace would be a main character. There could also be a lemon - grower or a merchant who deals with lemons. Maybe a young girl or boy who befriends the prince and helps him understand the significance of the lemons in their land.
Obviously, Ash and Lemon are the main characters. Ash is the one who does the betraying, and Lemon is the victim of that betrayal.
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.
Most likely Naruto himself would be one of the main characters. He's the titular character of Naruto. And probably some of his close friends like Sasuke and Sakura. From High School of the Dead, it could be Takashi Komuro.