Another possibility is a powerful entity within the Zerg hive itself. For example, a high - ranking Zerg that fears Percy will gain too much power or influence. This Zerg could use its position to undermine Percy at every turn, setting up traps and spreading false rumors about him within the Zerg community.
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 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.
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 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.
Well, perhaps Percy could start as a normal human or character and then through some strange event, like a scientific experiment gone wrong or a magical encounter, be transformed into a Zerg. He might then have to adapt to the Zerg's hive - mind society, learn their ways of war and survival, and face challenges from both the Zerg world and his former world which now sees him as a threat.
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.
It could be another ghost who is jealous of Casper's new human form. This ghost might try to sabotage Casper's attempts to fit into the human world, perhaps by causing chaos around him and making humans afraid of him again.