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. Since he is the one going to the past, he would be the central character. His younger self would also be important as he would interact with his future self.
Another plotline could be Naruto influencing the relationships between characters. He could try to bring Sasuke back to the village much earlier in the timeline, preventing a lot of the pain and conflict that occurred between them. Or he could help his parents, Minato and Kushina, in ways that would change the future of the entire Hidden Leaf village.
Obviously, Naruto and Kurama are going to be main characters. Since it's about Kurama going back in time, he'll be central to the story. Naruto will also be important as Kurama's jinchuriki and the one whose life Kurama is likely to impact. Minato might also be a significant character as he was Naruto's father and had a major role in the original story. And there could be appearances from other key characters like Sasuke, as Naruto's relationship with him was a big part of the original Naruto series.
The other main characters could include the Hokage of that time, like Hiruzen Sarutobi. Naruto might interact with him to try and influence the decisions of the village. Also, members of the Uchiha clan such as Itachi would play significant roles as Naruto could try to change their fates as well.
The main character is Naruto. In the past, he will surely encounter his parents. Minato and Kushina would be important characters in this fanfic. Also, Jiraiya, his mentor, would be a key figure. Since Naruto is going to the past, he might seek Jiraiya's guidance even more. And characters like Orochimaru would also be part of the story as Naruto may try to stop Orochimaru's evil plans earlier.
Sasuke is also very popular. As Naruto's friend and rival, many fanfictions explore how their relationship would be different if Naruto had a second chance. For example, Sasuke might be more open to Naruto's friendship earlier due to Naruto's new approach influenced by his future knowledge.
Well, Sakura is definitely one of the main characters, since it's about her going to the past.
Definitely Hermione. Since she's Harry's closest friend and very intelligent, she would be deeply involved in any changes Harry tries to make. Also, Snape. Harry's actions in the past could completely change his relationship with Snape. And of course, Voldemort. If Harry goes back far enough, he could potentially stop Voldemort's transformation into the evil wizard he became.
Sasuke is also highly likely to be affected. His relationship with Naruto and his own path of revenge could be drastically different. Maybe a small event that was supposed to bring them closer or push them further apart doesn't happen as expected, leading to a different dynamic between them.
The following are some classic novels about rebirth and returning to the past: " Great Country Heavy Industry ": This novel tells the story of a protagonist who traveled back to China in the 1980s and used his knowledge to participate in the country's industrial construction. 2. " Return to the Past and Become the Richest Man ": The story mainly revolved around a reborn protagonist who returned to his hometown after inheriting his family's wealth and started his own path to becoming the richest man.