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.
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.
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 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.
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.
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.
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 is often a popular character in Harry Potter demigod fanfiction. Her intelligence and thirst for knowledge can be easily combined with demigod - like powers. For example, she could be a demigod of wisdom, using her new - found powers to solve even more complex magical problems. Harry himself is also a common choice. As the Boy Who Lived, adding demigod powers to his existing abilities would make him an even more formidable character. And then there's Draco. The Slytherin could be a demigod with powers related to his family's dark secrets, which would add a whole new layer to his character.
Draco Malfoy is another character that often shows up in this type of fanfiction. He could be a rival pirate, constantly trying to outdo Harry. His family's wealth and influence in the wizarding world could translate into him having a powerful pirate ship or crew. There are also some fanfictions where Hagrid is involved. He could be a gentle giant pirate, perhaps having a special connection with magical sea creatures and using that knowledge to help the crew on their adventures.