Since it's fanfiction, the main characters could be the usual Naruto favorites like Naruto himself, Sasuke, or Sakura. But it could also introduce new characters created by the fanfiction author who play important roles related to chakra.
It's hard to say for sure without reading it. But if it follows the Naruto pattern, Naruto himself could be one of the main characters. He's often at the center of things in Naruto - related stories.
Sasuke could also be a main character. His Sharingan abilities could play a role in detecting or fighting the chakra monsters. Maybe he teams up with Naruto again to deal with this new threat.
Often, Naruto would still be one of the main characters. Since it's in the Naruto universe, he'd be likely to interact with the chakra gun in some way, either trying to master it or facing an enemy who wields it.
It depends on the specific fanfiction, but it could still feature Naruto, Sasuke, and Sakura as main characters. They might have new roles or developments related to the dark and light chakra concept.
Sasuke might also be a main character in such fanfiction. His Sharingan and his own strong chakra could potentially be used in relation to the chakra arrow. He could use his Sharingan to predict the path of the chakra arrow or enhance its power.
Sasuke is often a character associated with black chakra in fanfiction. Given his natural inclination towards power and his dark past, it's easy to imagine him being connected to such a powerful and mysterious form of chakra.
Often, Naruto himself might be associated with it. Since he has a large amount of chakra and is always looking for new ways to fight, the chakra bow could be something he discovers or is given. Another character could be Sasuke. His Sharingan might give him a unique advantage in using the chakra bow, like being able to accurately aim at distant targets.
In Naruto dark chakra fanfiction, Sasuke is frequently a central figure. Given his complex character arc and his inclination towards power - seeking, it's easy to envision him being affected by or exploring dark chakra. Naruto too can be a main character. His strong will and connection to positive chakra could be put to the test when faced with the allure of dark chakra. Another character often involved is Kabuto. He has a history of being involved in dark and underhanded schemes, so it's not a stretch to see him in stories centered around dark chakra. There are also some original characters created by fanfiction authors that are specifically designed to interact with the concept of dark chakra in unique ways.
In many naruto golden chakra fanfiction, Naruto is at the center. Kakashi could be an important character as he is Naruto's mentor. He might guide Naruto in controlling and using the Golden Chakra. Hinata could also be a main character, as she has deep feelings for Naruto. Her love for Naruto could be a driving force in some stories, either giving Naruto emotional support as he deals with the power of the Golden Chakra or even having her own role in helping him unlock more of its potential.
Sasuke could be another key character. There could be a plot where Sasuke is initially skeptical about Naruto's new chakra sage abilities but then comes to see their value in protecting the shinobi world. And Jiraiya, as Naruto's former mentor, might appear in flashbacks or even in some spiritual form to give Naruto advice on his chakra sage journey.