Obviously Naruto is one of the main characters since it's about him learning the truth. Sasuke could also be a main character as Naruto's relationship with Sasuke is often intertwined with the truth he's about to discover. Kakashi might be involved too, as he has a lot of knowledge about the past and could be the one to guide Naruto in his search for the truth.
Another important character could be Jiraiya. He was Naruto's mentor and knew a great deal about the secrets of the ninja world. His knowledge might be passed on to Naruto in some form, especially when it comes to the truth about certain powerful forces or the real intentions of some groups. And then there's Tsunade. As the Hokage, she has access to a lot of classified information and might play a role in Naruto's journey to learn the truth, either by directly revealing something or by being a source of conflict as Naruto tries to pry information from her.
Well, in many 'Naruto learns the truth' fanfictions, it could be about Naruto finally learning the truth behind his parents' death. Maybe he discovers some long - hidden secrets about the Uchiha clan that were concealed from him all along.
It could be Hermione. She is always so smart and observant. She might have stumbled upon some hidden information and decided it was time for Harry to know the truth.
Obviously, Harry is one of the main characters since it's about him learning parselmagic.
Percy is the main character. There could be a powerful and mysterious mage who serves as a sort of antagonist at first. This mage tests Percy's skills and knowledge of magic. But as the story progresses, they might end up having a more complex relationship, like a reluctant alliance when they face a common, greater threat.
In some fanfictions, there could be entirely new characters created just for the purpose of guiding Naruto in his journey to learn about his heritage. These characters could be mysterious hermits or powerful warriors from a forgotten era who hold the keys to unlocking Naruto's true identity.
It could be someone from within the village who doesn't want Naruto to learn ninshu. Maybe a powerful elder who fears that Naruto will use ninshu to change the traditional ways of the village and undermine their authority.
Obviously, Harry Potter is the main character as the title implies he is the one learning magic early. But there could also be Hermione and Ron. Hermione might be intrigued by Harry's newfound knowledge and Ron could be both a bit jealous and in awe of Harry's early magical abilities.
Well, Harry is surely a main character. There could be some other characters like Hermione and Ron who might be involved in his journey to learn the old ways. They could be helping him research in the library or having discussions about the old magic. Also, there could be some old ghosts in Hogwarts who have knowledge of the old ways and they play a role in this fanfiction as well.
Obviously, Harry Potter is the main character. He is the one who is learning the Dark Arts. Then there might be his friends like Hermione Granger and Ron Weasley. They could be either supporting his new path or trying to stop him. There may also be new characters created specifically for the fanfiction, such as a secretive Dark Arts master who takes Harry under his wing.
It could be that Dumbledore plays a more prominent role in this fanfiction. Since it's about 'the truth', perhaps Dumbledore has some secrets that are finally revealed, and he becomes a central figure in guiding Harry towards understanding. Additionally, some minor characters from the original series might be elevated to main characters if they hold key pieces of the truth.