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.
In some fanfictions, the little sister's friends at Hogwarts could be popular characters. They could be from different houses and have different personalities. For instance, a Gryffindor friend who encourages her to be brave and stand up for what's right, or a Ravenclaw friend who helps her solve the mysteries surrounding her brother.
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.
Obviously, Sherlock himself is a very popular character. His brilliant mind and unique personality are central to the fanfiction. The little sister, of course, is also a key figure. Depending on how she's written, she can be a strong, intelligent character in her own right.
Well, obviously the little sister herself is a very important character. Depending on how she's written, she could be really kind - like a Hufflepuff at heart - or really brave and adventurous like a Gryffindor. Then there are the classic characters from the original series that often play a big role. Hermione would likely be there to offer her wisdom and help the little sister with her studies. Ron would be the comical friend, always getting into some sort of trouble but also being very loyal.
It could be a rogue ninja from their clan who has a vendetta against the Uchiha family in general. Since Sasuke and Itachi are already well - known, the antagonist might see the little sister as an easier target to get revenge on the Uchiha. He could try to kidnap her or harm her in some way.
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.
Perhaps the fanfiction could focus on the little sister's own journey of self - discovery. Growing up in the shadow of her two powerful brothers, she might strive to find her own identity and power. She could train secretly and then surprise everyone with her newfound abilities. And in the process, Sasuke and Itachi might have to deal with their own feelings about their sister growing up and becoming independent.
Well, 'Sasuke's Little Sister's Journey' is also a notable one. It follows the sister as she embarks on her own adventure separate from Sasuke. She meets new characters, faces her own enemies, and discovers more about herself and her place in the ninja world. It's really interesting to see her character develop independently.