Sam Winchester as well. He could be part of the story, maybe trying to figure out what Castiel is doing in the past and how it impacts their current situation. His knowledge and research skills could play a role in understanding the consequences of Castiel's time - traveling.
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.
Often, other angels may appear. Angels from Castiel's past in Heaven might be part of the story, either as antagonists trying to stop Castiel from changing the past or as allies who support his mission for some reason.
One interesting plotline could be Castiel going back in time to prevent a major disaster in the past that has far - reaching consequences for the present. For example, he might stop a powerful demon from making a deal that would later lead to the death of many hunters.
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.
One common theme is self - discovery. As Castiel travels through time, he often learns more about himself, his purpose, and his place in the grand scheme of things. For example, in many fics, he realizes things about his past actions that he didn't fully understand before.
Rowena might also be a character in this fanfiction. She has a vast knowledge of magic. So, she could be the one who either accidentally turned Castiel into a dog in the first place or is the key to turning him back. Her sassy attitude would add a lot of flavor to the story as she deals with a dog - Castiel and the Winchesters.
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.
Inuyasha could be one of the main characters. Since he has a strong connection with Kagome in the original story, he might also be involved in the time - traveling adventure. Maybe he follows Kagome back in time or is already in the past waiting for her.
In addition to the Ghostfacers as a group, individual members like Ed and Harry could be considered main characters. They often lead the group's investigations. When they meet Castiel, their personalities clash in interesting ways. Ed might be overly eager to learn about Castiel, while Harry could be more cautious.
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.
Well, in many izuku goes back in time fanfictions, besides Izuku himself, his mother is an important character. He might go back in time to be a better son, protect her more, or even change things that affected her in the past. Another often - seen character is Aizawa. As Izuku's teacher, Izuku could use his time - travel knowledge to impress Aizawa earlier or get more help from him in his hero training. And of course, the villains like Shigaraki could also be involved. Izuku might try to stop Shigaraki from becoming a villain by going back in time and changing certain events that led to his fall.
Kikyo can be a prominent character as well. Her connection to Inuyasha's past is deep. In a time - travel scenario, she could be alive in the time Inuyasha travels back to. This could create a lot of drama as Inuyasha has to deal with his feelings for both Kagome and Kikyo in this new situation, and Kikyo might have a different reaction to Inuyasha's presence since she doesn't know all the things that have happened in the future yet.