There might be an assassin character who is the central focus. Maybe they have a vendetta against certain ninja clans in the Naruto world. And of course, Naruto or some of his friends like Sasuke could also be main characters as they interact with the assassin elements. For example, Naruto might try to redeem an assassin or oppose their evil plans.
I'm not entirely sure as it depends on the specific fanfiction. But it could include Naruto himself along with some newly created assassin characters.
It's possible that the main characters are a group of assassins who have their own motives and goals within the Naruto universe. They could be from a different region or have a special connection to an ancient power. And then there could be some Naruto characters who get drawn into their world, like Sakura who might be trying to understand and stop the assassins' activities if they pose a threat to the village or its people.
One interesting concept could be the unique abilities of the assassin characters within the Naruto universe. They might have special jutsu or skills that are different from the regular Naruto characters.
Obviously, the son of Kami would be a main character. And some Shinigamis would also be important. Naruto and his friends could also be main characters as they are part of the Naruto universe. They might interact with the son of Kami and the Shinigamis in different ways.
Often, there are also antagonist assassins who are main characters in a sense. They drive the plot by creating conflict. These assassins could have their own motives, like working for a powerful and shadowy organization. Their backstories, which might include things like being orphans trained from a young age to be killers, can add depth to the fanfiction.
In many Naruto Fox Assassin fanfictions, the Fox Assassin themselves might be the sole main character. Their journey of self - discovery, their battles, and their growth as a character are the central focus. However, in some cases, side characters like Kakashi could play an important role too. He might be the one who initially discovers the existence of the Fox Assassin or provides some guidance or opposition to the Fox Assassin depending on the plot of the fanfiction.
Sometimes, it could be a reimagined version of an existing 'naruto' character who discovers they have this assassin bloodline. For instance, Sasuke could be written to have a hidden assassin bloodline that awakens, leading to a whole new set of adventures and power - ups for him.
Since I don't know the specific 'assassin of black fanfiction', I can't say for sure. But it might have an assassin as the main character, perhaps with some side characters like the targets, allies, or rivals.
Haku is another character that is quite popular. Given his background as a ninja with assassin - like abilities in the original series, fanfiction writers can expand on his story. He could be part of a secret assassin organization or be a target of other assassins.
It could be the leader of the group that the assassin belongs to. They might be using the assassin for their own dark purposes and be the main force behind any evil plans.
It could be another powerful kitsune who wants to use the assassin for their own evil plans. This other kitsune might be jealous of the assassin's skills and wants to control them.
There could also be some characters unique to the fanfiction. Maybe a new demigod who has some special power related to Tartarus and ends up joining Percy on his quest. Or perhaps an ancient being who was imprisoned in Tartarus long ago and has knowledge that Percy needs to complete his mission.