Gary could be the main antagonist. He has always been Ash's rival and seeing Ash turn evil, he might feel responsible to stop him. Gary could gather his own team of powerful Pokémon and use his wits to oppose Ash at every turn.
The Spirit of the Millennium Items could also be the main antagonist. They were initially in harmony with Atem, but with his turn to evil, they might try to resist his control. Since they are powerful entities in their own right, they could become a force against Atem. They would not want to be used for evil purposes and would fight back against Atem's newfound malevolence.
" The Abominable " was a gangster film series directed by Takeshi Kitano, which included " The Abominable 1 - 2 " and " The Abominable: The Final Chapter." The series was set in the Japanese mafia, showing the struggles and complex interpersonal relationships between gangs. Takeshi Kitano was famous for his cold and dark style and violent and bloody scenes. He was considered a classic in gangster films. " The Evil Way: The Final Chapter " was the final chapter of the series. It was about the struggle and apology of the Japanese mafia Otomo on Jeju Island. The film was considered a classical gangster epic with high authenticity and artistic value. Overall, the series was loved and praised by audiences around the world.
It could be Yuno. Normally they are friends, but if Asta turns evil, Yuno might be the one to oppose him. Yuno has his own strong magic and a sense of justice that would make him a natural adversary.
The Guys in White could be the main antagonists. If Danny becomes evil, they might see this as their chance to finally capture and experiment on him. They would be constantly chasing him, which would make Danny see them as the main obstacle in his new evil plans, and thus they would be the antagonists in the story.
If Ruby turns evil, she could become her own antagonist in a way. Her former self, the Ruby that was good, could be seen as what she is now fighting against. Her friends would also become antagonists as they try to stop her from causing more harm.
The obvious choice could be the creator of the evil doll. Maybe it's a rogue Death Eater who has found a new and unique way to cause trouble at Hogwarts. They use the doll as their proxy to spread fear and chaos without being directly detected.
Dumbledore could be the main antagonist. Harry might see Dumbledore as having used him all along for some grand plan. If Harry turns evil, he would want to break free from Dumbledore's influence. Dumbledore, with his power and wisdom, would then try to stop Harry from causing too much harm, thus becoming the main opposing force.
It could be the Green Goblin. He might manipulate Spider Man in some way, perhaps by tricking him into believing that the only way to protect the city is to rule it with an iron fist. This would slowly corrupt Spider Man's good intentions.