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.
In 'pokemon ash betrayal fanfiction', Jessie, James, and Meowth from Team Rocket are often the ones to betray Ash in a different way. Instead of their usual comical attempts to steal Pikachu, they could form a more devious plan where they pretend to befriend Ash only to double - cross him later. Also, some fanfictions might have Professor Oak betraying Ash. He could be hiding some dark secrets about Ash's past or Pokémon in general and use Ash for his own experiments without Ash's knowledge.