The main antagonist could be a deity from another pantheon who is jealous of the Greek gods' influence. They target Percy's twin sister as a way to undermine the power of Poseidon and the other Greek gods. This deity could use their own powers and minions to try and stop her from fulfilling any prophecies or achieving any important goals in the Greek mythological world.
It could be an ancient monster that has a grudge against the family of Poseidon. Since Percy and his twin are children of Poseidon, this monster might target them specifically. Maybe it's a sea serpent that was once defeated by Poseidon and now wants revenge by going after his descendants.
Perhaps a powerful sorceress from an ancient line that has a long - standing grudge against the demigods. She sees the sister as an easy target at first, but as the story progresses, she realizes that the sister is much stronger and more determined than she thought. So she becomes more and more desperate to defeat her and stop her from fulfilling her destiny.
One possible main antagonist could be a powerful ghost who sees the combination of Danny and his twin sister's powers as a threat. This ghost could be trying to split them up or turn them against each other so that they can't use their powers together to stop his evil plans. Maybe he has some connection to their family's past in the Ghost Zone that makes him particularly interested in them.
It could be an old enemy of Percy's father, Poseidon. Maybe this antagonist blames Percy and his sister for something Poseidon did in the past and wants revenge on the family. So they target Percy's sister as a way to get to Poseidon through his children.
It could be someone who doesn't want Naruto and his sister to unite. Maybe a rogue ninja who fears their combined power. He might try to drive a wedge between them by spreading false rumors or causing misunderstandings.
It could be the warden of the prison. The warden might be a cruel and power - hungry person who wants to keep Percy imprisoned for his own reasons, like getting some sort of reward or power from those who put Percy there.
A powerful dark sorcerer could be the main antagonist. He might be seeking to use Percy's skills for his own evil purposes, and when Percy refuses, he becomes the target. The sorcerer would have a vast array of dark magic at his disposal, making him a formidable foe.
The character of Percy's twin sister can be developed by giving her unique personality traits. For example, if Percy is brave and impulsive, she could be more cautious and calculating. Her appearance could also be distinct, like having different colored eyes or hair. Additionally, her relationships with other characters in the Percy Jackson universe can help develop her. If she befriends Annabeth, for instance, it can show her intelligence and ability to form strong bonds.
It could be someone who hates the idea of Valkyrie and wants to stop Percy's mom from being a fan. Maybe a jealous god who doesn't like mortals being interested in Valkyrie.
Voldemort could be the main antagonist. He might have seen the potential in Harry's twin and decided to get rid of the twin to weaken Harry or use the twin for his own dark purposes.
It could be another sister. Sometimes in stories, sibling rivalry can be a strong source of conflict. The other sister might be jealous of the main sister's growth and change for the better and try to undermine her at every step.