Iruka sensei could be an important character affected. Naruto might show more respect and gratitude to him earlier, and Iruka might in turn support Naruto even more. Also, Tsunade. Naruto could help Tsunade with the village's leadership in a more proactive way, using his past - life experiences to prevent some of the problems that Tsunade faced in the original story.
Sasuke is definitely one. Since Naruto already knows Sasuke's path in his previous life, he could change Sasuke's fate. Also, Kakashi. Naruto might interact with Kakashi differently, sharing his future knowledge which could impact Kakashi's decisions.
Well, in the 'Naruto relives his life fanfiction', Naruto could potentially change the relationships in the Hidden Leaf Village. He might be able to reach out to characters like Sasuke in a different way, since he already knows Sasuke's pain and struggles from his first life. This could lead to a more harmonious bond between them much earlier in the story.
Sakura might be affected. She could be initially shocked by Naruto's new look and then gradually come to see it as a sign of his growth and independence. It might even make her re - evaluate her feelings for him.
Obviously, Harry Potter himself is the main character as the whole concept is about him reliving his life. His friends like Ron Weasley and Hermione Granger are also likely to be main characters as they are an important part of Harry's life in the original story and will surely play a role in this new scenario.
Obviously, Percy himself would be the main character. Annabeth would most likely be there too, as she is an important part of Percy's life. Grover would also be a likely character, being Percy's best friend.
Obviously Naruto himself is the most important as he's the one seeing the future. Sasuke would likely be there too, as he's Naruto's closest friend and rival. Sakura would probably also play a role, maybe as a skilled medical ninja in the future Naruto sees. Kakashi might also show up, perhaps as an elder guiding Naruto in his leadership role.
Iruka could play a role. He was Naruto's first teacher who believed in him. He might have some small piece of information that starts Naruto on his journey to find his parents. Then there's Tsunade. As the Hokage, she has access to a lot of secret files. She could either be an obstacle, not wanting Naruto to dig too deep, or she could be a helper, providing him with crucial information. And of course, the Nine - Tailed Fox. It was sealed inside Naruto by his father, so it might have some memories or knowledge related to Naruto's parents that it could share with Naruto.
His friends would be affected. For example, Sasuke might see Naruto in a new light and realize that they are more similar than he thought, both having their own inner struggles. Sakura would also be more empathetic towards Naruto and might try to be a better friend, offering more emotional support.
His friends like Sakura and Shikamaru would be affected too. With Naruto's changes, their relationships with him and with each other could be very different. For example, if Naruto changes the course of events, Sakura might not be as fixated on Sasuke in the same way. And Shikamaru might find himself in different strategic situations due to Naruto's interference.
Obviously Naruto himself is the most affected. He has to deal with the knowledge of the future and the changes he wants to make. Sasuke is also important. Naruto might try to change Sasuke's path to prevent him from going down the dark road of revenge. Kakashi could be another one. Naruto may share his future knowledge with Kakashi to make him a more effective teacher and ninja.
His friends would surely be affected. Hermione and Ron might be both amazed and a bit worried. They could be worried that this new power might change Harry too much or put him in more danger. Also, his enemies like Draco Malfoy would be affected. Draco might become more jealous or even more afraid of Harry.