Another option could be a character based on the author themselves, if they want to put a personal touch into the story. They could incorporate their own experiences, likes and dislikes into the little sister character. For example, if the author loves to paint, the little sister could be an amazing painter.
It could be an original character created just for the fanfiction. A girl with a spunky personality, who is full of energy and has a big heart. Maybe she has a unique style, like she loves to wear mismatched socks and big floppy hats.
It might be a journalist who is staying in room 317 while covering a story about One Direction. He's initially just there to do his job, but as he gets to know the band better, he becomes more of a fan himself and gets involved in their personal lives in ways he didn't expect. His perspective as an outsider - turned - insider could make for an interesting story.
One idea could be the little sister joining One Direction on tour. She could have her own mini - adventures at each stop, like trying local foods and making new friends. Another could be that she has a secret talent for songwriting and helps the boys come up with new hit songs.
Obviously, Harry Potter himself could be a main character. He could interact with the members of One Direction, like Niall Horan. Maybe Niall is a new student at Hogwarts and Harry is assigned to show him around.
Well, in this fanfiction, it could start with them finding the little girl in a difficult situation. Maybe she's an orphan. They then decide to adopt her. As time goes by, she becomes a part of their tours and experiences the world of fame with them. She might also have some insecurities at first, but with their love and support, she grows into a confident girl. It could be a really heart - warming story of how they become a real family.
A jealous witch who was the one who originally cast the curse on the Beast could be the main villain. She might see the little sister's presence as a threat to her plan of keeping the Beast trapped forever. Maybe she tries to trick the little sister into making things worse for the Beast.
A mischievous fairy who lives in their backyard could be an antagonist. She might be jealous of the attention the kids get and start causing trouble for the little sister. Like making her toys disappear or creating illusions to scare her. This would force the little sister to be brave and find ways to outwit the fairy with the help of her brothers (even if they don't initially believe her about the fairy).
It could be a new neighbor who is jealous of the little sister's relationship with her brothers. This neighbor might try to drive a wedge between them by spreading rumors or causing trouble. For instance, they could tell the brothers that the little sister doesn't really care about them and is just using them for her own gain.
In a 'little sister story', it's very likely that the little sister is the main character. But depending on the plot, there could be other important characters like her parents or an older sibling who also play significant roles in the story. For instance, if the story is about how the little sister is influenced by her older brother's actions, then both of them could be considered main characters in a way.
There could be a mysterious old neighbor. This neighbor might have some knowledge about magic or strange phenomena. When the little sister shrinks, the neighbor could offer some clues or tools to help her return to her normal size. Maybe the neighbor has had a similar experience in the past and can guide the little sister through her ordeal.
The parents would be important characters. They could have different reactions. Maybe the father is more accepting and tries to teach the giantess little sister how to use her new power for good, while the mother is initially worried about how society will view their daughter. Their relationship with the little sister and how they deal with the situation can be a big part of the story.