Villains from the Fairy Tail universe are also common. Since Natsu and his friends are often in conflict with bad guys, if his little sister is involved, she will also face these threats. Characters like Zeref or the various dark guild members can make appearances and create drama and action in the fanfiction.
There could be original characters created specifically for the fanfiction. For example, a magical creature that befriends Natsu's little sister and helps her on her journey. But the core Fairy Tail characters usually play important roles as well. They are the ones who provide the familiar backdrop of the Fairy Tail world that the story is set in.
Temari and Kankuro, Gaara's siblings, are likely to be in the fanfiction as well. They could play protective roles towards their little sister, or they might have some conflicts with her as she tries to assert her own identity. And there could be some antagonists, like rogue ninjas from other villages who see Gaara's sister as a target for some reason.
Obviously, Gohan and his sister are common characters. Goku might also be there as their father. Vegeta could be included too, as he is often part of the Dragon Ball storylines. Maybe Piccolo as well, since he has a connection to Gohan.
Hisoka's sister's friends or allies she makes along the way would also be common characters. They could be other up - and - coming Hunters or just ordinary people with special abilities. These characters help her in her adventures, whether it's in a fight or in solving a mystery. Another character that might appear is Ging, as he has a connection to the Hunter world and could potentially cross paths with Hisoka's sister in some way.
There could be other minor characters from Forks that get more spotlight in these fanfictions. For instance, the classmates of Bella's little sister who start to notice that there's something strange about her. They might become curious and start to dig deeper into the secrets of the town, which then draws in the vampires and werewolves.
The Weasley family would probably play a role too. Ginny could be a role model for the little sister, especially in terms of being a strong female witch. Fred and George might teach her some of their pranks or tricks. And Mrs. Weasley could be like a second mother to her, showing her the warmth of a family. The Malfoys could also be in the story, perhaps as antagonists, creating conflict for the little sister and Harry.
Often, there are the victims or potential victims of Jeff. They might be ordinary people in the neighborhood who start to notice strange things going on and suspect Jeff. Another common character could be a detective or a police officer who is trying to solve the mystery of the killings and might eventually cross paths with the little sister.
One interesting plot could be her going on adventures with Natsu and his friends. She might have unique magical abilities that help the group in their quests.
Erza is frequently paired with Natsu in these fanfictions. Given her strong and leadership - like qualities, she can take charge in agent - related situations, and Natsu can be the more impulsive force. Their interactions can range from Erza scolding Natsu for his reckless actions to them having deep conversations about their responsibilities as agents. Wendy is also a character that can be paired with Natsu. Her healing magic can be crucial during dangerous agent missions, and she looks up to Natsu, which can create a sweet and protective relationship in the fanfiction.
Sakura can also be a popular character in these fanfictions. She might be a friend to Sasuke's sister, or there could be some sort of rivalry or cooperation between them. Also, characters from other ninja clans might be involved, like Shikamaru. His intelligence could be used to help Sasuke's sister in strategic situations in the story.
Some fanfictions might introduce new characters that become popular. For instance, a new vampire who befriends the little sister. This vampire could have a different set of powers or a unique backstory that makes them stand out. Also, other members of the Cullen family, like Alice, could have a closer relationship with the little sister in the fanfiction, which makes her a popular character in this new narrative.