All - Might could also play a role in some 'izuku can see ghosts fanfiction'. He might be the one to first discover Izuku's ability or be the one to encourage Izuku to use it for good. He could provide moral support and also some practical advice on how to combine his ghost - seeing ability with his hero skills.
Obviously, Izuku is the main character. But often, his classmates from U.A. High School like Bakugo and Uraraka might also be involved. They might not be able to see the ghosts at first, but as the story progresses, they could start to believe Izuku and even help him deal with the ghost - related situations.
Another common character could be a ghost with a special connection to Izuku. This ghost might have been a former hero or someone with a strong will. Their relationship could be one of mentorship, with the ghost guiding Izuku in his journey, both in terms of his hero training and in handling his ability to see ghosts.
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.
Obviously, Izuku Midoriya is the main character as he is the one mastering One For All early. Katsuki Bakugo might also be prominently involved. Since he has a long - standing rivalry with Izuku, his reaction to Izuku getting such a powerful quirk early on could be a major part of the story. Maybe he feels jealous or challenged in a new way.
Hermione is often involved. She would be curious about Harry's new ability and might try to research more about it in the library. She could also help Harry figure out how to use this ability to their advantage in different situations.
Definitely his friends from U.A. High. Bakugo would be an interesting character in this scenario. He might initially be annoyed but then develop a soft spot for baby Izuku. Ochaco would also be there, using her quirk in creative ways to take care of him, like floating toys to keep him entertained.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.
Naruto himself is often a central character in 'naruto destruction fanfiction'. Since he is the main protagonist of the original series, fanfiction writers like to put him in situations where he has to deal with destruction. Sasuke is also frequently involved as his rival, and their relationship can be explored in the context of destruction, like if they have to work together to stop a greater threat of destruction.
Obi - Wan Kenobi is frequently in Anakin Snaps fanfiction. As Anakin's master, he has a big role to play. Their relationship is often explored, and Obi - Wan's reactions to Anakin's snapping are crucial. Sometimes he tries to bring Anakin back from the brink, while other times he might be the cause, albeit unintentionally, of Anakin's breakdown.
Well, Naruto is obviously a main character in most of these. But there could also be Sasuke. He might be either an ally or an adversary in relation to the dagger. For example, if the dagger has a power that can either be used for good or evil, Sasuke could be tempted by its darker potential.
There can be original characters created specifically for the fanfiction. For instance, a detective who is obsessed with bringing down the Joker but slowly starts to lose his own sanity in the process. This character can provide a new perspective on the Joker's reign of terror.
Obviously, the Reaper is a main character. Ruby Rose is also often involved as she is one of the central characters in RWBY and her connection to the concept of death (which the Reaper represents in a way) makes for interesting interactions. Weiss Schnee can also be seen in some fanfictions, as her family's past and her own story can cross paths with the Reaper's storyline in creative ways.