A new, mysterious trainer who befriends Ash only to steal his most prized Pokémon could be a betrayer. Ash would be heartbroken, especially if it was Pikachu. He would cry not only because of the loss of his Pokémon but also because of the broken trust in the so - called friendship.
Gary Oak could be a likely betrayer. He has always been Ash's rival, and in some fanfiction, he might take things too far and betray Ash in a crucial Pokémon battle, like using underhanded tactics to win. This would surely make Ash cry.
Another possibility is that it could be an old acquaintance who has changed for the worse. They might have been jealous of Ash and Lyra's success or popularity in the Pokémon world. This person could use their knowledge of Ash and Lyra's habits and weaknesses to betray them. It could also be a group of people rather than just an individual, like a team of thieves or poachers who see Ash and Lyra as obstacles to their illegal activities.
A jealous rival could be a likely betrayer. Maybe someone who is envious of Bell's popularity or his relationship with certain characters. For example, another adventurer who feels overshadowed by Bell might betray him to gain an advantage. Another possibility is a character who is being manipulated by a darker force. They might not have wanted to betray Bell originally but are forced to under the influence of some evil power.
In certain fanfictions, a Pokémon professor or an authority figure in the Pokémon world could betray Ash. They may have their own secret agendas. For example, a professor might use Ash's research data without his permission or manipulate him into dangerous situations for their own experiments. This would make Ash feel extremely betrayed as he looks up to these figures for guidance.
Team Rocket is a likely candidate. They're always up to no good and hypnotizing Ash would be just another devious plan for them. They could use it to finally capture Pikachu. Giovanni, as their leader, might also be involved in such a scheme. Another character could be a powerful and envious trainer who wants to eliminate Ash as competition by hypnotizing him and making him lose his skills.
It could be a fellow student who was secretly jealous of Harry all along. Maybe someone like Draco Malfoy. He had always been at odds with Harry and in a fanfiction, he could be pushed to the point of betraying Harry for his own self - centered reasons. Another possibility is a teacher. A teacher who was influenced by the dark side or had some hidden agenda could betray Harry. For example, if there was a teacher who was secretly working for Voldemort or a new dark wizard.
It could be some of the pure - blood supremacists who were biding their time. Even though the war was over, they might still resent Harry for his role in defeating Voldemort and decide to get revenge by making it look like he betrayed them. Another possibility is some of the Ministry officials who were corrupt and didn't want Harry's influence around.
Some fanfictions might have Misty betray Ash. Perhaps she gets lured by the promise of power or some valuable reward. She could be tricked into believing that Ash is no longer the friend she thought he was and that betraying him is for the greater good.
A possible betrayer could be a jealous demigod. There are always those in the Percy Jackson world who envy Percy's abilities and the attention he gets. This demigod might see an opportunity to get rid of Percy and make themselves look better in the eyes of the other campers.
His rival, Gary, could be a likely candidate. Gary has always been competitive with Ash, and in this fanfiction, his jealousy could reach a boiling point, causing him to betray Ash for his own gain, like trying to steal his Pokémon or get credit for his achievements.
Obviously, Ash and Lemon are the main characters. Ash is the one who does the betraying, and Lemon is the victim of that betrayal.