A potential main character could be an older, more experienced vampire who acts as a mentor or a threat to the black vampire. And then there could be a group of hunters. They would add an element of conflict as they try to hunt down the vampire while the vampire is trying to pursue love. This would create a complex web of relationships and power struggles.
The black vampire, obviously. They would be at the center of the story, with their own backstory, motives, and desires. There might also be a human love interest. This person could be someone who is initially unaware of the vampire's true nature and gets drawn into their world.
One possibility is a young and beautiful vampire girl and a brave human boy. The girl has been living in the shadows for years, and the boy is new to the town where the vampire resides. Their encounter changes both of their lives.
Well, I think the themes would be about the power dynamics. Vampires are generally powerful creatures, and in a love story, how power is balanced or unbalanced within the relationship is important. And of course, there's the theme of the 'otherness' that comes with being a vampire, especially a black vampire. It could explore how society views and treats this different being, and how love can either bridge or widen that gap between the vampire and the rest of the world.
Naruto and Shinso are the primary ones. But we could also have an old and wise vampire elder as a main character. This elder could have knowledge about a prophecy that involves Naruto and Shinso, and his actions and decisions would drive a lot of the plot as he tries to either fulfill or prevent the prophecy.
Obviously, the black wolf itself could be a main character. It could have a distinct personality, like being brave yet a bit solitary.
There could be a human girl who is not part of the main story but has some sort of special ability that Hanabusa is drawn to. Maybe she can see the future or has a unique form of psychic power. Their relationship would be complex as he is a vampire and she is a human, and they would have to deal with the differences in their natures.
One possibility could be Sebastian Michaelis, the demon butler, as the snake in a transformed state falling in love with Ciel Phantomhive. It would be a very strange and complex relationship as Sebastian's loyalty is supposed to be to Ciel in a more master - servant way, but if he were a snake in love, it would completely change the dynamic.
Since I don't know the specific 'as black as night fanfic', it's hard to say. It could be an original character created by the fanfic writer or it could be characters from an existing series that the fanfic is based on.
It could be a young protagonist who is brave but also has a lot to learn. This character might be the central figure in the story.
Well, of course, there's the little vampire himself. He could have a best friend who is a human, a brave kid who is not afraid of the vampire's nature. There might also be an old vampire mentor who teaches the little one about vampire traditions and how to control his thirst.
There is Yuki Cross. She is a central character. She has a complex relationship with the vampire world and the people in it. Then there is Zero Kiryuu, who also has his own issues and a deep connection to Yuki. Kaname Kuran is another main character. He is a powerful vampire and has strong feelings for Yuki as well.