However, it could also be a new character that is influencing Iris to act in a way that Ash doesn't like. Maybe there's a mysterious trainer who has been filling Iris's head with wrong ideas about Pokemon training, and Ash has to stand up not only to Iris but also to this new character's influence.
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.
Another possibility is that there is no real 'antagonist' in the traditional sense. Maybe it's just a difference in opinions between Ash and Iris. But since Ash feels the need to stand up to her, it could be that Iris's strong - willed nature is being portrayed as something that Ash has to face, like an obstacle in their relationship.
Well, it could be that Ash has finally had enough of Iris's overbearing behavior. Maybe she's been bossing him around about Pokemon training methods. So, Ash decides to stand up and tell her that he has his own way of doing things and that he's been successful with it. For example, Ash might have his own unique bond with Pikachu that doesn't follow Iris's ideas.
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.
In general, Ash is often depicted as his adventurous and determined self. He might be shown as being a bit naive at times. Iris, on the other hand, is usually portrayed as strong - willed and feisty. However, it's important to note that spank fanfiction is not a typical or widely - accepted form of fanfiction and may contain elements that are not in line with the original characters' personalities in a more official sense.
You should also include some foreshadowing. Drop little hints throughout the story before the big betrayal. Maybe Iris has been acting a bit strange lately, or there have been some small disagreements between her and Ash that gradually build up to the ultimate betrayal. This will make the betrayal seem more natural and less out - of - the - blue for the readers.
A powerful and greedy trainer who wants to capture Manaphy for its rare abilities could be the main antagonist. He might have a whole team of strong Pokemon and use underhanded tactics to try and get Manaphy.
It could be Team Rocket. Since they always cause trouble for Ash in the original series, in this fanfic, they might see Ash's transformation into a Pokémon as an opportunity to capture him and use him for their evil plans.
It could be a rogue Pokémon trainer who wants to capture Ash - now as Pikachu - for his own collection. This trainer might have a cruel streak and not care about the well - being of Pokémon.
Team Rocket could be the main antagonists. They might see Ash's transformation into Mew as a way to capture a powerful Pokémon - like being. They could devise all sorts of plans to catch him, thinking that they can use Mew - Ash's powers for their own evil purposes.
It could be a jealous trainer who doesn't like the attention that girl - Ash is getting. This trainer might try to undermine her at every turn during Pokemon battles.
It could be a trainer who takes advantage of Ash's loneliness. This trainer might trick Ash into giving up his Pokémon or getting into dangerous situations. For example, he could tell Ash that there's a special place where he can find friends if he follows him, but it's actually a trap.