Naruto is the key figure. Then there's Sakura. She could be part of the story where Naruto tries to change how she views him in the past. Jiraiya is another likely character. Since Naruto has learned a great deal from Jiraiya in the future, he might go back and train with him earlier or give him some important information. And of course, the Akatsuki members might be in the story as Naruto tries to prevent their evil deeds from the start.
The main character is Naruto. However, characters like Hinata could also play a role. Maybe Naruto goes back in time and starts to court Hinata much earlier. The other Konoha 11 members are also likely to be involved. They could be part of Naruto's plans to strengthen the village in the past. And the villains like Orochimaru and Pain are important as Naruto tries to stop their plans from ever coming to fruition.
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.
Obviously, Percy is one of the main characters. But there could be others depending on the specific plot of the fanfiction. Maybe there are some characters from Percy's past that play important roles.
Well, of course, Iron Man is often involved. His intelligence and technological know - how can be crucial in figuring out the time travel mechanics. Captain America is also a staple as his leadership and sense of right and wrong guide the team during their time - traveling adventures.
Obviously, Naruto is the central character. But often, his friends like Sasuke, Sakura, and Kakashi are also involved. Sasuke might be there to have some sort of reunion or conflict with Naruto. Sakura could be the one to heal Naruto if he comes back injured. And Kakashi, as Naruto's former teacher, would be there to welcome him back or assess his new abilities.
Well, Percy is obviously one of the main characters since it's about him getting sent back in time. There might also be his friends like Annabeth. In the past, he could meet characters who are important in the history of his world, like the ancient gods or heroes. They would play significant roles in his journey back in time as he might need their help or they could be obstacles in his way.
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.
The Joker himself is obviously the main character when it comes to his jealousy. Harley Quinn is often involved as she has a complex relationship with the Joker. Batman can also be part of the story in some cases. For example, if Batman is seen as a rival for Harley's attention in a strange way, the Joker might get jealous. Even though Batman is his enemy, the idea of Harley being interested in Batman in any way could trigger the Joker's jealousy.
Obviously, Mal is the central character. But often, her friends or romantic partners can be important characters too. For example, if she has a significant other, they will play a big role in how she copes with the pregnancy.
The main characters are Sam and Dean Winchester, of course. But in some fanfictions, Bobby Singer might also be involved. Since he was like a father figure to them, he could be the one guiding them through the time - travel process, or he could be there in the past waiting for them. Also, the characters of their parents, Mary and John Winchester, might show up if they go back to a time when their parents were alive.
Obviously, Harry is the main character. He's the one going back in time. Then there are the Marauders - James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew. Lily Evans, Harry's mother, also often plays a significant role as Harry might interact with her in the past. Dumbledore could be another important character, as he might be aware of Harry's time - traveling or be involved in some way in the events that occur in the past.
Neville Longbottom can also be a main character in some 'Harry Potter goes back in time' fanfictions. Given his growth and importance in the original series, in the fanfic, Harry might go back and help Neville become more confident earlier. This could have a significant impact on the fight against Voldemort as Neville could then be an even more powerful ally from an earlier time.