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.
Luna Lovegood is also quite popular in these fanfictions. She and the twin sister could bond over their shared belief in things that others might find strange. They might go on adventures in the Forbidden Forest together, looking for the Crumple - Horned Snorkack. And of course, Ron is always a key character. He could be like a big brother to the twin sister, and they could have a lot of funny moments together, especially when it comes to food at Hogwarts.
Severus Snape is often a prominent character in these fanfictions. His complex relationship with the Potter family could be explored further. He might have a different attitude towards the twin sister, perhaps seeing in her some qualities that remind him of Lily Potter. This could lead to some interesting interactions and plot developments, like him secretly protecting her or teaching her some advanced potions techniques. Dumbledore is also popular. His all - knowing and guiding role can be enhanced in the fanfiction, as he might have special plans for the twin sister in the fight against Voldemort.
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.
Hermione would still be a very popular character. She could be the one to help the twin sister study and understand the more complex aspects of magic. Her logical thinking and vast knowledge of magic would be invaluable in their adventures.
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.
Hermione Granger is often paired with Harry Potter's long - lost sister. Hermione's intelligence and knowledge of magic can complement the sister's unique abilities. They can form a strong trio with Harry, with Hermione providing the logical solutions and the sister bringing in her special powers.
Hermione would be a great character to include. She could be like an older sister figure to Harry's little sister, helping her with her studies and giving her advice on how to handle the challenges at Hogwarts.
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.
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.