Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
A common theme is the discovery of the sister's powers. For example, she might find out she can control water like Percy but in a different way. This adds an interesting twist to the story as they learn to use their powers together.
I really like 'A Sister's Tale in the Percy Jackson Universe'. In this fanfiction, the sister character brings a new dynamic to the story. It shows how she copes with the knowledge of the Greek gods and her relationship with Percy, which is full of both love and typical sibling spats. It's a very engaging read.
One great 'Percy Jackson and Sister' fanfiction is 'The Lost Sister'. It has an interesting plot where the sister has unique powers and they go on adventures together. The character development is really well - done.
The theme of growing up is prevalent as well. In these fanfics, the little sister has to grow up in a world full of danger and magic. She has to learn how to fight, how to make decisions, and how to deal with the consequences of her actions. This is often intertwined with the theme of self - discovery. As she grows, she discovers more about herself, her powers, and her place in the world of demigods.
You can start by giving her a distinct personality. Maybe she's really shy but has a fierce determination inside. For example, she might be quiet in group settings but when it comes to protecting her friends or family, she's fearless. Give her some hobbies too, like she loves to draw the different mythical creatures she's heard about. This can show her creativity and her connection to the world of the gods.
One idea could be that Percy's little sister discovers she has a unique power that even Percy doesn't have. For example, she can communicate with the spirits of the old gods that have long been forgotten. This could lead to her going on a secret adventure to find out more about these ancient deities and how they can help in the fight against the new threats.
For Percy, you can show his protective side more when it comes to his sister. Have him be more cautious and less reckless in situations where her safety is at stake.
One idea could be that his little sister discovers a new, hidden power within herself while on a quest with Percy. Maybe she can communicate with mythical creatures in a unique way that helps them solve a mystery.
Well, perhaps it implies that the Percy Jackson story has some characteristics typical of fanfiction. For example, it might have a certain playfulness or a deviation from the more common, established literary styles. It could also suggest that there are elements within the Percy Jackson universe that give the impression of being the product of a fan's wild imagination, like some of the unique creatures or the way the modern world and mythology are blended in a rather unorthodox way.