Obviously, Yusei himself. And his friends from his original time period might also show up in flashbacks or if he somehow brings them back in time with him. For example, Jack could be there.
There could be characters from the past era that Yusei travels to. Maybe some powerful duelists or important figures in the history of dueling. They could either be allies or antagonists to Yusei as he tries to navigate the past. Old masters of dueling, or even villains from a bygone era could be part of the story.
Obviously, Twilight Sparkle herself. And her friends like Rainbow Dash, Pinkie Pie, Fluttershy, Rarity, and Applejack might also be involved. Maybe they follow Twilight back in time or she has to work with past versions of them.
He could interact with the main characters from Duel Academy like Jaden. They might have some exciting duels together.
Maybe Yusei goes back in time to prevent a great disaster that happened in the past. He could use his knowledge from the future to change certain events and save lives.
Obviously, Josuke himself would be the main character. His friends like Okuyasu could also be involved. Maybe Okuyasu somehow follows Josuke back in time or Josuke has to find a way to bring him back if he gets left behind in the past.
In addition to the main characters, there could be some minor characters from the manga. For instance, the teachers at UA like Aizawa. Aizawa might notice that something is different about Deku and start to investigate. There could also be villains from the past that Deku has to face again, but this time with his future knowledge. Characters like Stain could be interesting to include as Deku might try to change the way he dealt with Stain in the original timeline.
Sokka would also be a common character. His strategic mind could be very useful in a different time period. He might be the one who figures out how to deal with the new situations they encounter while in the past. For instance, if they need to outwit an enemy in a historical battle, Sokka would be the one to come up with a plan.
Obviously, Albus Severus himself is the main character. His friends from his own time might also be involved in some way, like Scorpius Malfoy. And of course, the younger versions of well - known characters like Harry Potter, Hermione Granger, and Ron Weasley are likely to appear.
There are several potential characters. Aizawa could be one. Godlike Izuku could show up in the past and Aizawa might be suspicious of his powers at first but then come to realize his good intentions. Gran Torino is also a possibility. Izuku could learn different things from him in the past. And then there are the other pro - heroes. They might see Izuku as a strange new force but also recognize his potential to do good. His interactions with them could change the course of hero history in this fanfiction.
Some of Annabeth's other demigod friends from Camp Half - Blood might make an appearance. For instance, Grover could come to the school undercover to check on Annabeth and make sure she's okay in the mortal world. He could also help her out if there are any nature - related problems at the school.
Dean Winchester is often a common character. Since he has a strong bond with Castiel, he might be involved in the time - travel events either directly or through Castiel's actions in the past that affect him in the present.