The plot could revolve around her being kidnapped by a powerful enemy. Since she's Percy's twin, she might be seen as a valuable asset. Percy then has to go on a quest to rescue her. Along the way, he discovers more about their shared past and their parents' relationship. His sister might also have some secrets of her own that are slowly revealed during the rescue mission, like a special connection to an ancient prophecy that no one knew about before.
Maybe she has been living in a different part of the mythological world all this time. She could be part of a group that has a different view on the gods compared to the demigods at Camp Half - Blood. When she and Percy finally meet, they initially clash because of their different beliefs. But as they get to know each other, they realize they need to work together to face a common threat that endangers both of their worlds.
One possibility could be that she shows up at Camp Half - Blood unexpectedly. Maybe she was raised by a mortal family all this time, not knowing about her demigod heritage. When she arrives, she has powers similar to Percy's but also some unique ones, like the ability to communicate with sea creatures in a different way than Percy. This could lead to all sorts of adventures and conflicts within the camp as everyone tries to figure out who she really is and what her role will be.
Her character could develop in a way that is very much in contrast to Percy. While Percy is often impulsive, she could be more calculating. This difference causes some friction at first, but as they face challenges together, they learn from each other. For example, she might teach Percy to think before he acts in some situations, and Percy could show her how to be more spontaneous and trust her instincts. Over time, she becomes more well - rounded, balancing her cautious nature with a newfound sense of adventure.
One plot twist could be that they were separated at birth due to some prophecy. The prophecy might have foretold that if they were together, they would either bring great destruction or great salvation. So, the gods or some powerful force made sure they were separated. Another twist could be that their different upbringings have made them enemies unknowingly. Percy growing up with his mother and at Camp Half - Blood, and Clarisse at Ares' cabin. When they find out they're twins, they have to overcome their pre - existing grudges.
Well, one possibility could be that the sister discovers she has some unique powers that are different from Percy's. For example, she might be able to communicate with mythical creatures in a way that no one else can. This could lead to her getting involved in a quest to save a particular part of the mythical world that is in danger.
Another plot could involve the twin sister being cursed. This curse could be related to her connection with the sea as well. For example, every time she uses her powers related to the sea, she experiences a great deal of pain. Percy then has to find a way to break the curse. In the process, they discover an ancient prophecy that not only involves Percy but also depends on his sister's role in fulfilling it.
One possibility is that the sister has some unique powers that the Olympians fear. So they abandon her. In the fanfiction, she could be on a journey to discover her true self and her powers, while Percy unknowingly gets involved in her quest. Maybe she's being hunted by some mythical creatures because of her heritage, and Percy has to save her in the end.
One possible plot could be Percy feeling a deep sense of betrayal and loneliness. He might struggle with his self - worth as he wonders why his sister left him. This could lead him on a journey to find answers about his family and his own identity, perhaps even uncovering secrets about his demigod heritage that his sister knew and he didn't.
The sister could be under the control of a powerful god or goddess who has a grudge against Percy. She might be forced to betray him. For instance, she could lead Percy into a trap set by the god. She could lie to him about a mission, saying it's a rescue mission but in fact it's a trap where the god is waiting to take revenge on Percy for some past deeds. This would be a really heart - breaking betrayal for Percy as he trusts his sister.
One possible plot twist could be that Percy has hidden powers related to Kronos. Since Kronos is a powerful Titan, Percy might have some latent abilities that are similar but not as malevolent. For example, he could have a unique control over time on a small scale, like being able to slow down time briefly in a battle to gain an advantage. Another plot twist could be that the gods initially mistake Percy for being as evil as Kronos due to their relation. This would lead to Percy having to prove his loyalty to the gods and his friends all over again. He would face a lot of distrust and have to overcome many challenges to show that he is not like his 'twin' in nature.
Well, in a fanfiction like this, perhaps Percy's twin brother was separated from him at birth due to some prophecy or dark magic. The twin could have been raised by an enemy of the gods. As he grows up, he starts to question his loyalty to those who raised him when he begins to discover his true heritage. Eventually, he and Percy meet and have to reconcile their differences while also dealing with the threat that the twin's 'adoptive' side poses to the world of the gods.
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.