A group of power - hungry villains could also be the antagonists. They envy Percy's new - found Aku - like powers and want to take them for themselves. They plot against Percy, trying to steal his powers through various means. Percy has to not only deal with being in a new and strange form but also fend off these villains who are constantly after him.
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.
Another option for the main antagonist could be an ancient demon who sees Percy as Aku as a threat. Since Aku is a powerful force, this demon might want to destroy Percy in this new form before he gets too powerful. So, the demon constantly attacks Percy, which forces Percy to quickly learn how to use Aku's powers to defend himself.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
It could be a fallen angel who is jealous of Percy's new - found angelic status and wants to bring him down.
It could be a powerful leader from another race who sees Percy - the newly turned Zerg - as a threat to their own plans for domination. Maybe a human general who once knew Percy and now wants to destroy him because he has joined the Zerg side.
It could be a character who is against the idea of Percy being a girl. Maybe someone who believes in strict gender roles in their world and sees Percy's transformation as an abomination. For example, an old - fashioned wizard or a powerful deity.
There could be a group of poachers who are not just after regular game but are also involved in some dark magic. They oppose the hunters and especially Percy when he tries to stop their illegal and dangerous activities. They might have powerful backers or magic items that make them a formidable foe for Percy and the other hunters.
Perhaps it's one of Percy's former friends who has turned against him. This friend could be jealous of Percy's powers or has been influenced by some dark force. Their actions and taunts could be what drives Percy to become dark, making them the main antagonist in the story.
One possible plot could be that Percy, through some strange magical or scientific accident, starts to transform into Aku. At first, he might be confused and scared as he feels his body and mind changing. His friends would be shocked and try to find a way to reverse the process. But as he becomes more like Aku, he begins to gain Aku's powers, which he has to learn to control while still trying to hold on to his own moral compass.
It could be an ancient and powerful god who has long been plotting against Percy. This god might be jealous of Percy's power and popularity, so he devises a plan to turn Percy into a killer, knowing that it will lead to Percy's downfall and his own rise in power.
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.