Jiraiya is often a popular character in 'Naruto sent back in time fanfiction'. Since Naruto has a strong bond with him in the original series, in these time - travel stories, Naruto can learn even more from Jiraiya in the past and they can have new adventures together. Kakashi is also favored as he can be a mentor to Naruto in a different way with Naruto's new perspective.
Minato, Naruto's father, is a very popular character in this kind of fanfiction. With Naruto going back in time, he can finally meet his father and they can interact. Minato can also learn from Naruto about his future and the future of Konoha. And of course, the Akatsuki members can also be popular as Naruto tries to stop their evil plans from the start.
Well, Hinata is often a popular character in these fanfictions. Since Naruto has a chance to redo things, his relationship with Hinata can start earlier or be developed in a more in - depth way. Jiraiya is also a favorite. Imagine Naruto going back and being able to spend more time with Jiraiya, learning even more from him. And of course, Itachi. Naruto could potentially change Itachi's fate, and fans love to explore this possibility in the fanfiction.
Harry himself is obviously a very popular character in these fanfictions as he is the one being sent back in time. But also, characters like Sirius Black are often popular. Fans like to explore how Sirius would react to Harry being in the past, and how their relationship could develop differently.
Definitely Harry himself. Since he is the one being sent back in time, he is central to the story. His actions and decisions in the past are what drive the plot forward.
Obviously, Naruto himself is the main character as he is the one sent back in time. Sasuke is also often involved as Naruto's relationship with him is a central part of the Naruto story. Kakashi could be involved too, as Naruto might seek his help or try to influence his training methods. Also, the various Hokages might be important characters as Naruto could interact with them in the past to change the course of events.
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.
One interesting plotline could be Naruto using his future knowledge to prevent certain tragedies. For example, he might stop the Uchiha massacre by warning the clan or finding a way to resolve the conflict between them and the village earlier. Another could be him training his younger self more effectively, leading to a much stronger Naruto at an earlier age. He could also change the way he interacts with characters like Sasuke, maybe preventing Sasuke from leaving the village in the first place.
One interesting plotline could be Naruto using his future knowledge to prevent the Uchiha massacre. He might be able to change things in a way that doesn't lead to Sasuke's downward spiral. Another could be him training his younger self to be stronger faster, so that he can face the early threats in the Naruto world more effectively.
One possible reason could be to correct some mistakes in the future. Maybe there were some events that led to a really bad outcome, like the destruction of the Hidden Leaf Village, and Kami thought sending Naruto back in time could prevent it. Another reason might be to give Naruto a chance to learn more about his predecessors and gain new powers or knowledge that could help him in his original timeline.
Obviously Naruto himself would be the central character. His friends like Sasuke and Sakura would also be involved. Iruka - sensei would play an important role as Naruto's teacher at the academy again. And characters like Kakashi might also make an appearance, perhaps to give Naruto some special training or advice based on his 'second chance' at the academy.
Percy Jackson would be an obvious choice. His skills as a demigod and his experience with different quests would make him a great character to be sent back in time. He could use his water powers in unique ways in different historical settings.