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- school values of the Avengers, and in a more modern, cynical world within fanfiction, some might see his ideals as outdated and turn against him.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.
Well, Ares could be involved. He has had his clashes with Percy in the original stories. In fanfiction, his war - like nature and desire for chaos could make him set up Percy to look like he's betrayed the community. Also, Octavian from the Roman side might be a character in such a scenario. He's power - hungry and jealous of Percy's status. He could spread false rumors or create situations that make it seem as if Percy has betrayed the Roman demigod community.
The village elders are often depicted as the ones involved in Naruto's betrayal. They might see him as a threat to their power or the stability of the village. Also, some fanfictions might involve Danzo. He has always been a bit of a shadowy figure in the Naruto series, and his underhanded ways could easily be written into a plot where he is behind Naruto's false accusations and subsequent banishment.
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.
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.
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.
Cinder Fall could be a likely betrayer. She's already a villain in the original series, and in a fanfiction, she could pretend to be an ally to one of the RWBY members only to betray them at a crucial moment, perhaps leading them into a trap set by Salem.
Sasuke could also be a character who is frequently betrayed in these fanfictions. Given his complex personality and his history of being both a loner and a seeker of power, he might be lured into dangerous alliances in the DxD world. And his partners in the DxD world could betray him when they realize they can't control him or when his power becomes a threat to them.
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.
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.