In some fanfictions, an ancient and evil spirit could be the antagonist. This spirit might have been sealed away for a long time, and when Ash accidentally releases it, it targets him for mind control. It wants to use Ash as a vessel to regain its full power and take over the Pokémon world. And there are also some power - hungry scientists or researchers in the Pokémon world who might see Ash as a perfect test subject for their mind - control experiments.
Team Rocket is very likely. They've always been after Ash and his Pokémon. Using mind control on Ash would be a new way for them to get what they want. They could make Ash give them Pikachu or other strong Pokémon. Another one could be Giovanni. As the leader of Team Rocket, he has the resources and the motive to use mind control on Ash to further his own evil plans in the Pokémon world.
There are also powerful and rogue Pokémon that could be antagonists. For instance, a dark - type Pokémon with strong psychic abilities. This Pokémon might be jealous of Ash's popularity or his success in battles. So it uses its mind - control powers on Ash to disrupt his life and his Pokémon journey. Also, some new and mysterious trainers who have a grudge against Ash might use mind - control techniques they've learned from some forbidden knowledge.
One type of antagonist could be a corrupt Pokemon League official. In the fanfic, this official might bribe Ash to do something unethical, like throwing a battle against Iris. When Iris finds out, she'll feel betrayed. Another likely antagonist is a Pokemon hunter. If the hunter captures a Pokemon that Ash and Iris both care about, and Ash, in an attempt to rescue it, makes a deal with the hunter that seems to go against Iris's values, she'll feel betrayed. And sometimes, it could be a self - centered Pokemon trainer who wants to be the only star. They might set up Ash and Iris to fight each other and make it look like one has betrayed the other.
Melisandre is a likely candidate. She already has some mysterious powers in the original series, so in fanfiction, it's easy to imagine her delving deeper into mind control. She could use it to sway Stannis Baratheon more effectively or even target his enemies.
Bonnie could be a likely candidate. She's a powerful witch. She might use mind control to protect her friends. For example, if a dangerous vampire is after Elena, Bonnie could use mind control to make the vampire forget Elena exists and move on to another target.
Another possibility is Bulbasaur. It has always been a loyal and steady companion to Ash. During the trip, it could see how far they've come together and confess its gratitude for all the experiences and growth they've shared.
One likely antagonist could be a powerful new villain who has the ability to mind - control. This villain might target Deku specifically because of his potential as a hero. Another could be an old enemy of the hero society who sees Deku as a key to their revenge.
One common plot is when a villain tries to mind control Ash to use him against his friends. For example, Team Rocket might get their hands on a new device that can control Ash's mind. Then they could make him battle his own Pokémon against other trainers in a cruel way. Another plot could be an evil Pokémon with mind - control abilities targeting Ash. This Pokémon might be under the control of a more powerful and mysterious force.
One possible main antagonist could be a powerful and arrogant trainer who wants to prove that he is better than Ash and Nora. He might try to steal their Pokemon or sabotage their battles.
A Titan who has escaped from Tartarus could be a potential mind - controller. Titans are powerful beings, and one of them might use mind control to try and regain their former power. For example, if a Titan could control Percy Jackson, they could use his abilities to start a war against the gods. Also, a powerful nymph who has been mistreated by demigods might use mind control to get back at them. She could manipulate Percy's mind to make him do things that would harm the demigod community.
Gary Oak is a likely character. He has always been a rival to Ash and in fanfiction, he might betray Ash out of jealousy or a desire to be the ultimate Pokémon trainer. Another could be Misty. Maybe she gets influenced by some external force or her own insecurities and turns against Ash. Brock could also be a candidate in some stories. If he falls in love with a girl who has a vendetta against Ash, he might be forced to choose between his loyalty to Ash and his new love interest.
Perhaps a group of bandits who steal Pokemon and sell them on the black market. They see Ash and Serena as threats because they are trying to stop their illegal activities. These bandits could be led by a really tough and mean - looking guy who has no mercy when it comes to getting what he wants. They would constantly be trying to sabotage Ash and Serena's attempts to protect the Pokemon and the people in the medieval setting.