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.
John Watson often makes an appearance. His role as Sherlock's friend and partner can be further developed in these fanfictions. He might be the one who first notices the little sister's potential or he could be the mediator between Sherlock and his sister when they have their differences.
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 explore the little sister's backstory. Maybe she was sent away for some reason when she was younger and now she returns, bringing with her secrets from her past. This could add an element of mystery and drama to the story, as Sherlock and Watson try to figure out what she's hiding and how it relates to their current cases.
She can be a catalyst for change in fanfiction. Her presence might disrupt Sherlock's usual way of doing things. For example, if she suddenly shows up in his life, he may have to reevaluate his solitary lifestyle. In some stories, she could be the key to solving a particularly difficult mystery that Sherlock has been struggling with, perhaps because she has access to a different social circle or has a different way of thinking.
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.
Obviously, Sherlock Holmes himself is a central character. His complex personality makes it interesting to see him in a romantic context. He's often portrayed as initially cold and distant but gradually warming up to love.
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.
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.
In fanfiction, Sherlock Holmes' little sister often serves as an interesting addition to the Holmesian universe. She can be portrayed in various ways. For example, she might be highly intelligent like her brother, with a unique approach to solving mysteries. Sometimes, she's a bit of a rebel, challenging the norms of Victorian society that her brother also tends to defy in his own way. Fanfic authors use her character to explore different aspects of the Holmes family dynamic and to create new and exciting storylines.
A common theme is family relationships. It often shows how the sister's relationship with Sherlock is complex, with elements of rivalry and deep affection. Another theme is her own detective skills, highlighting how she can be as sharp as Sherlock.
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.