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.
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.
Obviously, Harry Potter himself is a central character in such fanfiction. Then there's Hermione Granger. Her intelligence could play a big role if she also travels back in time. Another popular one is Severus Snape. His complex relationship with Harry and his past make him an interesting character to include in time - traveling scenarios.
One interesting plot could be Harry going back in time and preventing certain tragedies from happening in the wizarding world, like stopping Voldemort's rise before it really began. Another could be him changing the relationships between characters, for example, making Snape and James Potter become friends early on.
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.
One interesting plot twist could be that Harry goes back in time and accidentally changes the sorting of some of the characters. For example, Draco Malfoy could end up in Gryffindor instead of Slytherin, which would completely change their relationship and how events unfold in the wizarding world.
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.
One popular fanfic in this vein is 'Time's Twist for Harry's Bottom'. It explores the unique concept of Harry being sent back in time due to a magical mishap involving his bottom (in a rather comical way). Another one could be 'Back in Time, Bottom First' which has an interesting plot where Harry has to deal with the changes in the past while his bottom - being the source of the time - travel has some strange side - effects.
One common theme is self - discovery. Since Harry is sent back in time, he gets a chance to see his past self and the world in a new light, which often leads to him learning more about his true nature. For example, in some fanfics, he realizes his hidden strengths or weaknesses. Another theme is the alteration of the timeline. His presence in the past can change small or large aspects of the future he came from, and the fanfics usually explore the consequences of these changes.
In some fanfictions, there could be a magical creature that has somehow found its way into the orphanage. For example, a house - elf might be there, either sent by someone to protect Harry or just lost. This house - elf could be a key character in the story, helping Harry with small tasks and providing some comic relief. There may also be a character from the Ministry of Magic who is sent to check on Harry's well - being in the orphanage, and this person could have their own motives and secrets.