Naruto is obviously the most common character since it's his time travel. Hashirama Senju is also very common as he was the founder of Konoha and has a huge influence in the Senju clan's history. Tobirama Senju often appears too as he was an important figure in the clan and in the development of Konoha.
Some original characters created within the Senju Clan in fanfiction can also become popular. For example, a young Senju prodigy who has a special ability that was never seen before in the clan. Writers can explore this character's journey to master their power and their place within the clan and the ninja world.
Definitely Maka. She's always at the forefront of trying to make things right in the Soul Eater story, so it makes sense for her to be a time - traveler in fanfiction. Black☆Star could also be a character who time - travels. His ambition to be the greatest might lead him to go back in time to gain more power or change the past to prove his superiority earlier. And Crona might time - travel in an attempt to escape their tortured past or to prevent the events that made them so broken in the first place.
Sakura could be a common supporting character. She might assist Naruto in his mission to save Rin. For example, she could use her medical ninjutsu to heal any injuries. Kakashi is also likely to be involved. He has a deep connection to Rin, so he would be a key figure in the events.
The Shinigami is a key character as well. It's the one that enables the time travel in the fanfiction. Other characters like Jiraiya might also be involved. Jiraiya could play a role in training Naruto in the past, using Naruto's future knowledge to enhance the training and create new jutsus together.
Danzo could also be a common antagonist. He was very power - hungry and had his own dark secrets. In a time - travel scenario, he might view Kakashi and Naruto as threats to his control over the Hidden Leaf Village in the past. He could try to eliminate them to maintain his grip on power.
In Yugioh 5ds time travel fanfiction, Yusei Fudo is a very common character to be involved in time travel. He is the hero of the story and has a strong will to protect his friends and the city. His connection to the Signer Dragons also gives him a special role in any time - related events. Another one is Akiza Izinski. Her psychic powers could potentially be related to time travel in some fan concepts. Also, she has a complex backstory and emotions that could be explored through time - traveling scenarios. And of course, Bruno, with his mysterious origin and knowledge, is likely to be involved in time travel plots as he might hold the keys to understanding the time - space continuum in the Yugioh 5ds universe.
Naruto himself is often a popular choice. Fans like to see him go back in time and use his future knowledge and skills to change things. Sasuke is also quite popular. His complex backstory and powerful abilities make for interesting time - travel scenarios. Another popular character is Kakashi. He has a lot of mystery and potential for development in time - travel stories, especially with his Sharingan and his relationship with Obito.
Sasuke is also a popular character in these fanfictions. In time travel scenarios, he might either be an ally or an antagonist to Naruto depending on the plot. For example, if Naruto's time travel changes Sasuke's past in a way he doesn't like, Sasuke could oppose Naruto.
Naruto himself is often a central and popular character. Since it's a time - travel story about him, his antics in the past are what drive a lot of the humor. His naivete in the past combined with his future knowledge can lead to some really funny situations.
Sasuke is also popular. His brooding nature and his own complex journey make him a great character to include in time - travel plots. Fans like to see how he might be influenced by a time - traveler, especially if it affects his path towards revenge or redemption.