In such a fanfic, perhaps Ash and Paul were on a journey together in the Pokémon world. They might be part of a team for a big tournament. Then, one of them could make a deal with a rival group behind the other's back for personal gain, like getting access to a rare Pokémon. This would be the betrayal.
It could be that Ash discovers some sort of secret about Harrison that he misinterprets. Instead of talking to Harrison about it, he takes actions that seem like a betrayal. For instance, Ash might tell others about this secret, thinking it's for the greater good, but it actually causes a lot of harm to Harrison.
Perhaps in the fanfic, a new and mysterious Pokémon arrives. This Pokémon is able to manipulate the thoughts of those around it. It makes Mewtwo believe that Ash is a threat to all Pokémon. So Mewtwo starts to act against Ash. Ash, on the other hand, is clueless as to why Mewtwo has suddenly turned on him. He has to dig deep into the secrets of this new Pokémon and find a way to reverse what it has done to Mewtwo.
It's mainly Ash and Paul. But depending on the plot of the fanfic, there could be other trainers or characters involved. For instance, a new trainer who comes in between them and somehow instigates the betrayal. Or a Pokémon professor who might unknowingly play a part in the events leading to the betrayal.
The 'ash and paul betrayed fanfiction' could center around a variety of themes related to betrayal. It might be set in a world where their goals and ambitions clash, leading to one feeling betrayed by the other. For example, if they both have different ideas about how to train Pokémon or achieve victory in the Pokémon world. Paul might be very competitive and see Ash's more friendly approach as a betrayal of the 'true' way to be a Pokémon trainer. And Ash could feel betrayed if Paul were to, say, steal an opportunity from him in a crucial moment.
In this kind of 'ash and riolu betrayed fanfic', it might be that Ash and Riolu are in a Pokémon tournament. They meet a new trainer who seems friendly and offers to team up with them. However, during the tournament, this so - called friend betrays them by using underhanded tactics. This could include things like secretly weakening Riolu before a battle or spreading false rumors about Ash to other trainers. As a result, Ash and Riolu not only lose the tournament but also feel deeply hurt and betrayed.
One possible plot could be that Ash and Misty share a kiss, which is witnessed by Paul. Paul, feeling out of place or perhaps jealous, decides to leave. After Paul leaves, Ash and Misty might have to deal with the aftermath of their kiss. They could face mixed reactions from their friends, and they themselves might be confused about their feelings towards each other. Maybe they start to explore a romantic relationship further.
Well, it could be that Ash makes a discovery that goes against Arceus' grand plan and is thus betrayed. Maybe he finds out something about the balance of the Pokemon world that Arceus doesn't want known.
Perhaps May betrays Ash because she's influenced by a new group of trainers. They might convince her that Ash's way of training Pokémon is wrong or that he's holding her back from achieving greater things. So, she sides with them against Ash. This would show how easily friendships can be tested in the competitive world of Pokémon training.
Perhaps Pikachu discovers a 'dark truth' about Ash. For example, Pikachu might find out that Ash has unknowingly been involved in something that goes against Pokémon ethics, like being associated with a group that mistreats Pokémon. This discovery could make Pikachu feel betrayed by Ash first, and then Pikachu's actions would make Ash feel the same way in return.
In this fanfiction, it might be that Ash has been friends with Lemon, but then Ash sides with an enemy of Lemon's for some false gain. He might think that the enemy can offer him something better, like power or wealth, and in doing so, he stabs Lemon in the back by giving away Lemon's plans or weaknesses to the enemy.