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.
It's also possible that the main antagonist is the one who framed Percy in the first place. They might be outside the prison but still have influence inside, like having bribed some of the guards to make sure Percy stays locked up.
Perhaps there is a rival group or another inmate who sees Percy as a threat even within the prison. This antagonist might try to make Percy's life even more difficult while he's imprisoned, or even try to stop him from escaping.
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.
It could be a character who wants to use Percy - turned - Aku for their own evil purposes. Maybe a dark wizard who thinks that Aku - Percy can be the ultimate weapon to take over the world. This wizard would try to manipulate Percy's new form and powers to do his bidding.
A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.
A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.
The main antagonist might be a new character, a powerful being that rules the realm where Percy is banished. This being could see Percy as a threat to his or her power and try to stop Percy from finding a way back.
A powerful sorcerer could be the main antagonist. He might be after Percy's powers or trying to use the school as a base to spread chaos. Maybe he wants to open a portal to the underworld right in the middle of the schoolyard.
A group of rogue demigods could be the main antagonists. They might be jealous of the attention the daughter gets because of her parents. So, they plot to make her life difficult and try to drive a wedge between her and the other demigods.