Well, if the fanfiction stays true to the original in terms of character inclusion, characters like Hide might still be important. His relationship with Kaneki, who is now a girl, could be really interesting. Maybe he has a different kind of protective instinct towards her, or there could be some romantic undertones that were not present in the original when Kaneki was male.
Obviously, Kaneki as a girl would be one of the main characters. Depending on the plot, other characters from the original story like Touka might also play important roles. They could have different relationships with Kaneki now that she's a girl. For instance, their friendship or rivalry might change in unexpected ways.
In 'kaneki is a girl fanfiction', aside from Kaneki herself, the antagonists from the original story could also be main characters. Their interactions with Kaneki as a girl could be different. For example, they might underestimate her because she's a girl, but then be surprised by her strength and determination, which could lead to some intense confrontations and plot twists.
Obviously, Kaneki himself would be the central character. But also, his friends like Hide could be involved. Hide was always there for Kaneki, and if Kaneki goes back in time, he might want to protect Hide better from the start. Touka could also be a major part. Maybe Kaneki goes back to a time when he first met Touka and tries to change their relationship for the better, and together they could work towards changing the fate of ghouls.
There could be a wise old wizard who becomes Kaneki's mentor in the new world, guiding him on how to use the magic there.
Tatsumi from 'Akame ga Kill!' would be a great pair. Tatsumi is a fighter who has a strong sense of justice, and his interaction with Kaneki, who has his own moral struggles as a ghoul, could create an interesting dynamic. They could fight together against common enemies in a crossover world.
Well, the blueberry girl herself is obviously a main character. She's usually the center of the story with her unique traits. There might also be a sidekick, like a little talking animal that accompanies her on her adventures.
Without having read the 'bastards girl fanfiction', we can only guess. The main character is likely the girl who is identified as a 'bastard'. She may be accompanied by a cast of other characters. There could be a group of outcasts like her who form a kind of surrogate family. There might also be antagonists, perhaps those who uphold the social norms that marginalize her. And then there could be characters who are initially indifferent to her but later become important in her journey, like a kind - hearted noble who starts to see her worth beyond her 'bastard' status.
Since it's named 'the sunlight girl fanfiction', the girl herself is bound to be a main character. There could also be other characters like her friends or family members who support her or are part of her journey. Maybe there's an antagonist as well, someone who is opposed to her because of her sunlight - like qualities, perhaps someone representing darkness or jealousy.
Obviously, Fireheart as a girl would be the main character. She would be the center of the story, with her actions and decisions driving the plot forward.
Well, in such a fanfiction, one interesting idea could be how Kaneki's character traits change when presented as a girl. Maybe she'd have a different approach to relationships, like being more empathetic in a different way compared to when she was male. Also, her appearance as a girl might lead to new interactions with other characters in the story.
In a crossover with 'One Piece', Blackbeard would be an interesting character to pair with evil Kaneki. Both are complex and morally ambiguous characters. Blackbeard's thirst for power and his ability to manipulate situations could be enhanced by Kaneki's ghoul powers. In the 'Fairy Tail' universe, Jellal could be a good match. Jellal has his own dark past and his powerful magic could interact in an exciting way with Kaneki's kagune. They could either be rivals or form an uneasy alliance.
Maybe there's a mysterious visitor who comes to the ward from time to time. This person seems to know something about the girl's past or has some connection to her that she doesn't remember at first. As the story unfolds, this visitor becomes a key figure in the girl's life in the ward.