Who could be the main antagonist in a 'Percy has a sister' fanfiction?

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.

Perhaps the main antagonist is a dark figure from within Konoha itself. This person doesn't want Naruto's family to be reunited because it could expose some dark secrets about their own actions in the past. They would do anything to keep the sister hidden and Naruto in the dark, even if it means causing harm to both of them.

It could be a vengeful god. Maybe a god who feels that the sister's existence is an insult to their power. So they set out to stop Percy from finding her. For example, Hades might be the antagonist if he believes that the sister's return could disrupt the order in the Underworld.

Another possibility is a new, unknown force from the Underworld. Maybe Hades has lost control over some dark entity that has grown in power and now aims to take over Olympus. This entity could be extremely powerful, with abilities that the demigods have never faced before. It would be a great antagonist as it would force the Percy Jackson characters to adapt and find new ways to fight in order to save Olympus.

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.

Maybe the main antagonist is an ancient evil that has awakened. It has the power to manipulate Artemis's mind, making her kidnap Percy against her will. This evil entity wants to use Percy as a sacrifice to gain more power. So, while Artemis is the one who physically kidnaps Percy, the real force behind it, the ancient evil, is the true antagonist.

It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.

Another option is a group of anti - magic fanatics. They view Percy's wings as an abomination because they are a result of magic. They hunt Percy down, believing that by getting rid of him and his wings, they are protecting the world from the 'evils' of magic.