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.
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.
In a Percy Jackson baby sister fanfic, she could have some really cool adventures. Maybe she discovers she has her own unique powers early on, like being able to communicate with sea creatures in a different way than Percy. She could also get into trouble by wandering into the wrong part of the camp, like the restricted areas where the more dangerous magical items are kept.
Her power could be related to healing. Since the demigods often get injured in their battles and adventures, having a healer in the family would be a great advantage. She could use her power to heal not only Percy but also other demigods. This would make her very popular in the camp and also attract the attention of the gods. Some gods might want to use her for their own purposes, while others might want to protect her because of her valuable gift.
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.
Yes, in some Percy Jackson fanfics, he could be depicted as a Titan. Fanfic writers often take creative liberties and might come up with unique storylines where Percy has Titan - like qualities or is even reimagined as a Titan for the sake of an interesting plot twist.
In some fanfics, Percy and Annabeth as siblings might have a relationship filled with protectiveness. Percy could be the over - protective older brother, always looking out for Annabeth. He'd be there to defend her in any dangerous situations within the world of Greek mythology that they inhabit. For example, when facing mythical creatures, he'd put himself in harm's way first.
One great Percy Jackson little sister fanfic is 'The Adventures of Percy's Little Sister'. It shows her journey in the Camp Half - Blood, facing the same challenges as Percy but with her own unique perspective. Another one could be 'Percy's Sister: A Hidden Power'. It focuses on the little sister discovering a power that no one expected her to have.