I don't know specifically as it depends on the fanfiction. It could be Naruto and his friends if the 'ninth fist' is something they encounter or develop.
There's a good chance that if it's a Naruto - centered fanfiction, Naruto, Sasuke, and Sakura could be main characters. Maybe Naruto discovers the 'ninth fist' and his friends help him master it or they face challenges together because of it. But it could also introduce new original characters created by the fanfiction author who are directly related to the 'ninth fist' concept.
The main characters in 'naruto ninth fist fanfiction' could vary greatly. It might focus on an older generation of characters from the Naruto series who have some connection to the 'ninth fist' in the past. Or perhaps it centers around a group of rogue ninja who are after the power of the 'ninth fist'. Without reading the fanfiction, it's hard to say for sure, but it's likely to include some familiar faces from the Naruto franchise along with some new creations to drive the story forward.
Sometimes, an original character created specifically for the fanfiction could be a main character. This character could be a mentor - like figure who teaches Naruto about the dragon fist. They might have a deep knowledge of the technique's origin and its true potential, guiding Naruto on his journey to master it.
I'm not entirely sure as there could be many interpretations. It might be a fan - made story that involves a unique fighting style or technique related to Naruto, perhaps a new form of the 'ninth fist' that the author has created within the Naruto universe.
Naruto himself is often very popular in 'Naruto Fist' fanfiction. Since it's related to a technique named after him, writers like to explore new aspects of his character in relation to this. He might be shown teaching others the Naruto Fist or coming up with new variations of it.
Most likely, Naruto is the central character as the title implies he's the one creating the Trickster Fist. His friends like Sasuke, Sakura, and Kakashi could also be important characters. Sasuke might be intrigued or even a bit jealous of Naruto's new technique. Sakura could be worried about the risks Naruto takes while developing it. And Kakashi might offer some guidance or be surprised at Naruto's innovation.
Rock Lee could be a great main character. He's known for his taijutsu skills, and adding the Explosive Fist to his repertoire would make for an exciting story. He could use it to overcome his limitations in ninjutsu and genjutsu and prove himself even more as a powerful ninja.
It could be a rogue ninja who was once a student of a great fire - style master but turned evil. He wants to use the power of the 'fire fist' to destroy all the ninja villages and rule over the world.
The main characters would include the ninth human, who is likely to drive the story forward. Then there are the monsters that the ninth human encounters. Sans could be a main character as he often has a deep understanding of the situation in the Underground and his interactions with the ninth human could be crucial. Papyrus, with his cheerful and naive nature, would also be an important character as he might befriend the ninth human easily. Toriel, being a motherly figure, could also play a major part in the story, perhaps trying to protect or guide the ninth human.
Danny Rand is the central figure in the Iron Fist original story. He is the protagonist who undergoes the journey to become the Iron Fist. Colleen Wing is an important character as well. She has her own martial arts skills and she and Danny form a connection. Another character is Ward Meachum, who has a complex relationship with Danny, sometimes being an adversary and sometimes having a more nuanced role in the story.
Since I don't have a full knowledge of the 'Ninth House Novel', the main characters might be those who are deeply involved in the affairs of the ninth house. They could be leaders within the house, or those who are on a quest to discover something important about it. Maybe there are some outcasts or rebels who play a crucial role in the story as well.
It's possible that Ron could play a major role. He might be the one who first discovers the connection between the two universes and tries to convince Harry and Hermione to explore it further. Another potential main character could be a new student at Hogwarts who has a secret past related to Fist of the North Star. This student could introduce the martial arts and the different philosophy of Fist of the North Star to the wizarding community. Also, Dumbledore could be a main character in a sense that he is aware of the existence of the Fist of the North Star world and is guiding the students to use the knowledge from there for the good of the wizarding world.