A new, original character could be the main character in a Naruto Explosive Fist fanfiction. This character could come from a small, unknown clan that has secretly been preserving the knowledge of the Explosive Fist. They journey to the Hidden Leaf Village to learn more about the ninja world and end up sharing their unique technique, the Explosive Fist, with the other ninjas there.
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 an original character created specifically for the fanfiction. Maybe a young ninja who has a natural talent for explosives.
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.
One idea could be a young ninja discovers a new form of the Explosive Fist technique. He then has to train hard to master it while facing challenges from rival clans who want to steal the technique. Another could be a story where a character uses the Explosive Fist to save a village from a powerful enemy that no one else could defeat. Maybe they combine it with other jutsus in unique ways.
Maybe Tsunade. She is known for her love of sake. She could have some hidden knowledge of the Drunken Fist. Since she is also a very powerful ninja, she could train Naruto in both the physical and strategic aspects of the Drunken Fist. She could show him how to use his chakra more effectively while performing the drunken - like maneuvers.
There's a possibility that it's a spirit or a ghost of a past master of the drunken fist. Naruto somehow makes contact with this entity, and the spirit decides to pass on its knowledge to Naruto, guiding him through the process of mastering the drunken fist in a rather supernatural way.
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 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.
Another possibility is that an old hermit who once studied the Gentle Fist in the past but is not directly related to the Hyuga clan could help Naruto. This hermit could have a different approach to the technique, one that is more spiritual. He could teach Naruto how to connect his inner chakra with the natural energy around him to enhance the power of the Gentle Fist. This way, Naruto would not just be recreating the technique in a traditional sense but also evolving it.
It could be Hinata. Since she is a master of the Gentle Fist in the Naruto universe, she would be a logical choice to teach Naruto. She has a deep understanding of the technique and also has a close relationship with Naruto, which would make for an interesting dynamic in the fanfiction.
In this 'naruto explosive master fanfiction', it could center around a group of characters who discover ancient explosive techniques. These techniques are so powerful that they attract the attention of both good and bad forces in the Naruto world. The explosive master character, whether it's an existing Naruto character or a new creation, has to protect these techniques from falling into the wrong hands. Along the way, they might learn about the history of these techniques and how they were once used to either create peace or cause great destruction in the past.