In a lot of 'naruto chakra scalpel fanfiction', Sasuke can be a main character related to it. He has great control over his chakra and his Sharingan could potentially give him an edge in using the chakra scalpel in more advanced ways. For example, he could use his Sharingan to predict the best places to use the scalpel on an opponent's body during a fight. Also, there are often original characters created by the fanfic authors who are centered around the use of the chakra scalpel, with unique backstories and abilities related to it.
Sakura is often involved. Since she's a medical ninja, it's natural for her to be associated with the chakra scalpel in fanfiction. She might be seen using it in new and innovative ways to save lives or in battles against strong opponents. Another common character is Naruto himself. Even though he's not primarily a medical ninja, in some fanfics, he could learn to use the chakra scalpel in his own unique way, perhaps combining it with his massive chakra reserves and shadow clone jutsu.
Kakashi is also a character that can show up in 'naruto chakra scalpel fanfiction'. He's a very skilled ninja with a wide range of abilities. His knowledge and experience could be used to teach others about the chakra scalpel. He might also use it himself in some fan - created scenarios, like in a stealth mission where the precision of the chakra scalpel comes in handy for silently taking out enemies or disabling traps without making much noise.
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.
Naruto himself is often involved. Since he is the main character of the original series, it's natural for him to be part of fanfictions related to the chakra fruit. He might be the one to discover it or have to protect it from villains.
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.
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.
Obviously Naruto is the main character as the golden chakra cloak is related to him. Sasuke might also be involved as his rival. Kakashi could play a role as Naruto's mentor, guiding him in controlling this new 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.
Sasuke can also be a central figure. His quest for power in the original Naruto makes him a prime candidate in fanfictions about godlike chakra. He might either gain it on his own or be in a rivalry with Naruto over it, leading to intense battles and power struggles.
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.
Obviously Naruto himself is a main character. He is the one with the pacemaker so he is central to the story. Sasuke might also be involved. He could be initially skeptical of Naruto's new situation with the pacemaker but then come around to help. Sakura too, as a medical ninja, she could play a crucial role in understanding and perhaps even trying to modify the pacemaker.