Definitely the protagonist trainer. They are usually in the center of the action and have many relationships. A so - called 'friend' trainer could betray them by spreading false rumors about their training methods, causing them to lose respect among other trainers. Then there are Pokémon that have been with a trainer for a long time. If the trainer suddenly decides to abandon them for a different type of Pokémon, these long - time companions would feel the sting of betrayal. And let's not forget about the Pokémon breeders or researchers in the fanfiction. They can also be betrayed by their colleagues if their research findings are stolen or misused.
The main trainer character is very likely to experience it. They might trust a rival who then does something underhanded, like releasing their Pokémon. Pokémon themselves can also be characters that feel the pain. For example, if a Pokémon is traded away without its consent, it would surely feel betrayed.
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.
Snape is a likely candidate. In the original series, his loyalties were always a bit of a mystery at first. In fanfiction, he could be further explored in terms of false betrayals or real ones. Another one could be Draco Malfoy. He was on the verge of becoming a full - blown Death Eater in the original, so in fanfiction, he could either truly betray his Hogwarts classmates or be wrongly accused of betrayal.
Professor Sycamore could be a character involved in a surprising way. Maybe he discovers something about Ash that he believes is dangerous and tries to use Serena to stop Ash. Or perhaps there's a misunderstanding where Serena thinks she's doing the right thing by following the professor's orders which seem to go against Ash. It shows how complex the relationships can be in these fanfiction scenarios.
Their closest friends like Brock and Misty would surely be affected. They've been through so much with Ash, and his betrayal would be a huge shock. And for Lyra, her friends in her own region might be deeply hurt.
One common theme could be the broken trust between trainers. For example, a trainer might betray their partner by stealing a rare Pokémon. Another theme could be the emotional turmoil of the betrayed Pokémon. It might feel abandoned and angry. Also, the idea of revenge often pops up, where the betrayed character seeks to get back at the one who betrayed them.
Peter Pettigrew is a very likely character to be involved in more acts of betrayal in fanfictions. Since he already betrayed James and Lily Potter in the original story, it's easy to imagine him continuing his traitorous ways. Another could be Draco Malfoy. In some fanfictions, he might be made to do more extreme acts of betrayal rather than just being a bully. He could be forced to betray his schoolmates in a more serious way for the sake of his family's safety.
Cedric Diggory could be an unexpected character in a betrayal scenario in fanfiction. Maybe in an alternate version, he becomes jealous of Harry's fame from being in the Triwizard Tournament and betrays him. This could be a really interesting twist as Cedric is generally seen as a good and honorable character in the original story.
Sasuke is a likely candidate in some fanfictions. Given their complex relationship throughout the original series, it's easy to imagine a scenario where Sasuke's thirst for power or some misinformation leads him to betray Naruto. Another could be Sakura. If she becomes jealous of Naruto's growing power or if she's manipulated by outside forces, she might turn against him.
Naruto himself could be a victim of family betrayal. His father Minato or mother Kushina might be involved in some alternate - universe plot where they turn against him for some dark reason. Sasuke's family is also a likely candidate. Itachi's actions could be re - imagined in a way where his real motives are more about betrayal of the Uchiha family rather than protecting it. And Hinata's family might be involved too, like if Neji, before his death, was made to betray the Hyuga family in some fan - created scenario.
Captain America could be a likely character to be betrayed. He represents the old - school values of the Avengers, and in a more modern, cynical world within fanfiction, some might see his ideals as outdated and turn against him. Also, Tony Stark. His wealth and sometimes over - confident attitude could make some of the other Avengers, especially those with more humble backgrounds, feel resentful and potentially betray him. And Thor, being an Asgardian, might be betrayed due to the other Avengers' mistrust of his alien nature or the potential threat Asgard could pose.