Even Ginny could be written as a betrayer in a fanfic. For example, if she becomes possessed by a dark force, she might turn against Harry. Her relationship with Harry is so central in the original story that a betrayal from her would be a huge shock and create a lot of drama in the fanfic.
In 'harry potter is betrayed fanfic', Luna Lovegood might be a character who betrays Harry. Maybe she is under the control of some mind - altering spell or has been deceived by a false prophecy that makes her think she is doing the right thing by betraying him. Also, Hagrid could be a shock betrayer if he is somehow forced to choose between Harry and his beloved magical creatures, and chooses the latter in a way that is seen as a betrayal to Harry.
One character that might betray Ichigo is Chad. In some fanfic scenarios, if Chad is corrupted by some dark force or if he discovers some 'truth' about Ichigo that he believes is a threat to the world, he could turn on him. Also, Orihime has the potential to be a betrayer in fanfictions. If she is brainwashed by an enemy, like a powerful Hollow or an evil Quincy, she could be made to act against Ichigo. Her powers are unique and if misused, could cause a lot of harm to Ichigo.
Ron is a likely candidate. In the original story, they had their ups and downs, and in fanfiction, it's easy to imagine him doing something that Hermione would see as a betrayal, like not supporting her in her academic pursuits. Another could be Draco Malfoy. He could pretend to befriend her for some devious reason and then turn on her.
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.
Obviously, Harry Potter himself would be one of the main characters. Since the title implies his betrayal by the light, he is the central figure through which the story unfolds. Hermione and Ron could also be important characters as they are Harry's closest friends. Their reactions to Harry's situation and whether they side with him or the light side that is betraying him can add a lot of drama to the story.
Even Hinata could potentially be written as a betrayer in some really out - there fanfictions. Maybe in a crossover where there are mind - control elements, Hinata could be forced to turn against Naruto. But this would be a very different take on her character as she's usually portrayed as extremely loyal to Naruto.
Gary could be a likely character. He has always been Ash's rival and there could be a plot where his jealousy or some other motive leads him to betray Ash. Another could be one of his so - called friends from the Pokemon League who might be influenced by power or some dark secret. Or, in some really dark fanfictions, it could be a Pokemon that Ash trusted deeply but for some reason turns on him.
Luke could be a likely candidate in some fanfictions. He already had his own issues and motives in the original story, and it wouldn't be a stretch for a fanfiction writer to take that further and have him betray Percy in a more elaborate way.
In a crossover, characters from the other franchise might betray Danny. For example, if it's a crossover with a show about vampires, a vampire character who seems friendly at first could betray Danny for his ghost powers. They might want to use his powers for their own dark purposes. Another likely candidate could be a character who is jealous of Danny's abilities. Maybe in a crossover with a superhero world, a sidekick who feels overshadowed by Danny's powers and popularity might be the one to betray him.
It could be Ron. Maybe he starts to feel that Harry has changed too much after the war and is no longer the friend he once knew. He might be influenced by outside forces or his own insecurities. Another possibility is Hermione. If she gets too involved in the new Ministry politics and starts to believe that Harry's actions are not in line with the new wizarding order, she could turn on him.