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.
Apollo, in his demigod form, would be an interesting character to send back in time. He could use his musical and healing abilities in different ways. Also, his knowledge of the past as a god could add a lot of depth to the story, even if he has to deal with being in a more limited demigod form in a different time period.
One idea could be that the demigods are sent back to prevent a great disaster that was actually caused by one of their own kind in the past. They have to figure out who it was and stop it without changing too much of the future.
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.
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.
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.
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.
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.
It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.
The theme of identity is also common. When demigods go back in time, they may have to hide their true identities to fit in or face challenges because of their different nature. Also, there's often a sense of adventure. They are in a new, yet old, world and they have to navigate through different cultures and situations, which makes for an exciting read in 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.