Well, the sister herself is obviously a main character. And perhaps her best friend who might be the only one she has confided in about her secret.
It could be the sister and her older or younger sibling. If it's an older sister, the younger sibling might be the one who discovers the secret by accident. Or if it's a younger sister, the older one might be trying to figure out what's wrong with her all this time.
The sister and a stranger who somehow gets involved in her secret. For instance, if her secret is about a lost item that has a great sentimental value and a stranger finds it and tries to return it, which then gets the sister worried about her secret being exposed.
I don't know for sure. But I assume the sister is one of the main characters. There might also be other family members like her parents, brothers, or other sisters who could be important in the story since it's about a sister's secret.
One main theme could be self - discovery. She might be on a journey to find out who she really is, away from the expectations of her family and society, and this process is full of secrets.
It could involve a secret friendship. She might have befriended someone who was considered an outcast in society, and she didn't want her family to know because of the potential judgment. So she maintained this friendship in secret throughout her life.
The theme could also be about personal growth. The sister might have overcome some great obstacle in secret, like dealing with an illness or a difficult situation at school or work. And this secret experience has made her grow as a person. It could also be about friendship, where she has a secret friendship that has influenced her in ways her family doesn't know, and this friendship is an important part of her real - life story.
It could be her long - lost sister. They might be reconnecting through texts after years of being apart, and the mom wants to keep it a secret until she figures out how to tell the rest of the family about this newfound relationship.
Obviously, the main characters are the brother and the sister. But there could also be other minor characters like their pet dog that might be involved in their secret somehow.
Lucy herself would surely be one of the main characters. And maybe she has a best friend in the story, like a character named Lily who helps her through all the challenges.
Since I haven't read it, I can't say for sure. But likely the sisters would be main characters, perhaps their parents or other family members could also be important characters in the story.
A librarian and a student could be the main characters. The librarian has access to the secret files and notices the student's interest in a particular topic. As they start to explore the files together, love blossoms. The librarian can provide a lot of background knowledge about the university's history and the secrets within the files.
A teacher at her school could also be an important character. The teacher might notice the girl's strange behavior due to her secret and try to help her. This could lead to a series of events where the girl has to decide whether to trust the teacher and reveal her secret or keep it hidden.