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.
The relationship could start with Percy and Annabeth being overprotective parents. Percy might be constantly worried about the baby's safety near water, while Annabeth would focus on teaching the baby knowledge and wisdom. As the baby grows, they could start to have little adventures together. For example, the baby could go on a mini - quest with them around Camp Half - Blood, which would strengthen their bond.
One idea could be that Percy's baby sister discovers she has a unique power that even Percy doesn't have. For example, she might be able to communicate with the spirits of the ancient Greek heroes in a way that gives her special knowledge. Another plot could be that she gets kidnapped by a new enemy who wants to use her to get to Percy. Percy then has to go on a dangerous quest to rescue her, facing all kinds of mythical creatures along the way. A third idea is that she is actually a reincarnation of an important figure from Greek mythology, and as she grows up, she starts to remember her past life and the secrets it holds, which causes a stir in the demigod world.
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.
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.
Give her a distinct personality. For example, she could be really curious, always asking questions about the demigod world. Make her brave but also a bit naive at times. She might be more in tune with nature than other characters, able to sense things about the environment. Another aspect is to show her relationship with Percy. She could look up to him and want to be just like him, but also have her own ideas about how to handle situations.
The baby might be a key to a long - forgotten mystery. It could be related to an ancient pact between the gods. Percy finding the baby could be a sign that something big is about to happen in the world of the demigods. The baby's identity could be slowly revealed as Percy and his friends investigate. It could be a symbol of hope or a harbinger of doom, depending on the circumstances surrounding its appearance.
Another plot could revolve around the challenges of raising a demigod baby. Percy and Annabeth would have to protect the baby from various mythological threats. They might face monsters that are after the baby because of its potential power. Also, they would need to figure out how to educate the baby about their demigod heritage in a world that is both dangerous and full of wonder.
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.