Sasuke could also be involved. Given his thirst for power, he might be more likely to explore the realm of dark summons. He could be on a journey to find the most powerful dark summon to exact his revenge or to achieve his goals of changing the ninja world, which would lead to many conflicts and plot developments.
Naruto himself could be a main character. He might be tempted to use dark summons to gain power to protect his village or friends. For instance, in some fanfictions, he discovers a dark summon that could potentially defeat a powerful enemy threatening Konoha, but at a great cost.
The Nine - Tails also plays a major role. In a dark Naruto story, the Nine - Tails could be more of a corrupting influence, whispering dark thoughts into Naruto's mind and fueling his descent into a more menacing figure in the ninja world.
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 a main character. He's usually the one who stands up against the dark guild's evil plans. Sasuke might also be involved, as his power and sometimes ambiguous moral standing can add interesting dynamics when dealing with the dark guild.
Obviously Naruto himself is a main character as he is the one under the dark control. Then there would be his friends like Sasuke and Sakura. Sasuke might be crucial in trying to understand and counter the dark power since he has his own powerful abilities.
Naruto himself is often involved. He might be either tempted by Dark Hadou or have to fight against those who use it. Sasuke could also be a main character in such fanfictions. His quest for power could lead him to explore the Dark Hadou. And then there's Kakashi, who with his experience and knowledge, might be involved in trying to stop the spread of Dark Hadou.
The butterfly itself could be considered a main character in a sense. It has its own personality and abilities. It could communicate with Naruto in a unique way, like through telepathy. The relationship between Naruto and the butterfly would be central to the fanfiction. It could be a source of strength for Naruto and also have its own goals and desires within the story.
There can be original characters created specifically for the fanfiction. For instance, a detective who is obsessed with bringing down the Joker but slowly starts to lose his own sanity in the process. This character can provide a new perspective on the Joker's reign of terror.
Varian is, of course, the main focus. However, characters like Eugene could also be involved. He might be in a more antagonistic role towards Varian in this type of fanfiction, trying to protect Rapunzel from what he perceives as Varian's dangerous actions.
The Dark Seducer themselves are obviously a main character. Often, the player character from Oblivion might be involved as they interact with the Dark Seducer. Another could be Mehrunes Dagon, as the Dark Seducer has ties to the Daedric Princes.
Well, obviously Dark Silver himself is a main character. Often Sonic might also be involved as he is a central figure in the Sonic universe. Their interactions can create interesting storylines, like a rivalry or an uneasy alliance.