An organization that believes in using Pokemon for dark magic could be the main bad guy. They see Iris and her Pokemon as a threat to their plans, so they constantly try to stop her. They might have their own powerful, corrupted Pokemon that they use to battle Iris. They could also be stealing Pokemon from other trainers to use in their dark experiments.
Perhaps a rival trainer who has a long - standing grudge against Iris. He's jealous of her skills and the bond she has with her Pokemon. So, he sets out to undermine her at every turn. He might even go as far as to try and steal her Pokemon or spread false rumors about her in the Pokemon - training community.
It could be an evil sorcerer. This sorcerer might be after Noctis because of his special powers. He sees Noctis as a threat to his plan of ruling the world. The sorcerer could also target Iris because she has a pure heart, which he wants to corrupt, and Lemon because Lemon has a map that leads to a powerful magical artifact that the sorcerer desires.
Most likely, Iris could be seen as the main antagonist in this fanfic. Since the title says 'ash stands up to iris', it implies that Iris is the one causing some sort of problem that Ash needs to oppose. It could be her attitude or her actions that are creating conflict.
It could be a group of Team Rocket - like villains. They are after the unique powers that the reborn pokemon might possess. Since the reborn pokemon has a human - like intelligence, they see it as a valuable asset to use for their evil plans, like stealing all the precious pokemon resources or taking over a certain region.
It could be a super intelligent pirate who has been secretly controlling events in the One Piece world. This antagonist might be several steps ahead of everyone, creating elaborate plans to take over the Grand Line. Luffy, with his newfound intelligence, has to match wits with this foe.
The main antagonist could be a mean-spirited mole. This mole is constantly digging tunnels under the garden and causing chaos. He doesn't like Gnomeo and Juliet's love because he believes it's disrupting the natural order of the garden. He tries to separate them by causing misunderstandings between their families.
An organization of rogue scientists who want to experiment on Pokemon could be the main antagonist. They might be trying to create super - powerful mutant Pokemon or find a way to control all Pokemon. If you're reborn as a Pokemon, they might capture you for their experiments. This could lead to a story where you have to escape from their labs and expose their evil plans to the rest of the Pokemon world.
It could be a powerful and evil Pokemon trainer who wants to capture Ash in his new Pokemon form. This trainer might be after Ash because he believes that a human - turned - Pokemon has some special power or value.
Maybe a powerful and evil pokemon trainer who has a vendetta against Ash. Now that Ash is in a vulnerable pokemon form, this trainer could try to capture and control him. The trainer might have some dark motives related to using Ash's unique situation to gain power in the pokemon world.
Maybe there's a rogue Pokemon trainer. This trainer is jealous of Dawn's skills and the attention she gets. When Dawn joins Team Rocket, this trainer sees it as a chance to expose her as a fraud. He or she then tries to stop Dawn and Team Rocket's activities, thinking that they are up to no good, which they are, but the trainer has his own self - centered motives.
It could be a rival politician who sees Trump's association with Grimace as a threat. This rival might try to spread false rumors about Grimace being a dangerous or untrustworthy being, just to undermine Trump.