He could betray them by breaking a promise. For instance, if he promised to keep a family member of the Wesleys safe but failed to do so.
If Harry were to use the Wesleys' trust to gain access to something valuable and then use it for his own purposes rather than the family's, that would be a form of betrayal. For example, if they trusted him with a magical artifact for safekeeping and he used it for his own power - up instead.
Perhaps Harry betrays the Wesleys by siding with their enemies in a conflict. This could be because he was deceived into believing the enemies' cause was just, or he had no other choice in a complex situation.
Well, in certain fanfiction scenarios, Ron's jealousy could be a factor. He might feel overshadowed by Harry for so long that it finally drives a wedge between them, and Hermione, being in a relationship with Ron, gets caught up in it. Another possibility is that a prophecy or some ancient magic forces them to make choices that seem like a betrayal to Harry, but they believe they are doing it for the greater good.
In certain fanfic ideas, Arceus could be influenced by an evil force. This evil force manipulates Arceus into believing that Ash will cause great harm in the future. So, Arceus betrays Ash as a preventive measure, according to the fanfiction plot. However, this is just a fictional concept and not part of the official Pokémon storyline.
Well, in some fanfiction scenarios, Percy Jackson might betray the Avengers due to a misunderstanding. Maybe he's been misinformed about the Avengers' intentions towards the demigods. For example, if he hears false rumors that the Avengers plan to control the demigods, he could turn against them in a fit of protectiveness for his kind.
In some fanfictions, Percy might betray Chaos if he discovers that Chaos's plans are more harmful than he initially thought. For example, if Chaos intends to destroy all of creation and remake it in a way that Percy believes is wrong.
It could be due to a misunderstanding. Suppose Ron overhears a conversation between Harry and Hermione that he misinterprets completely. He might think they are plotting against him or don't really consider him a true friend. So in a fit of anger and hurt, he decides to betray them.
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.
Well, she could betray Percy by revealing his secrets to his enemies. Maybe she's forced to do so under some kind of magical compulsion or threat.
She could betray her mentor. In many fanfiction stories, a mentor - student relationship is crucial. If Fem Daisuke betrays her mentor, it could be a huge turning point. Maybe she steals an important item or knowledge from her mentor that was meant for a greater good and uses it for her own selfish purposes.
Hermione could be the one. Maybe she gets jealous of Harry's fame and power, and starts spreading lies about him, which leads to his betrayal. As a result, Harry feels so hurt that he turns to Voldemort.
Another possible betrayer could be a jealous friend. For example, Neville might feel overshadowed by Harry throughout the war. After the war, this jealousy could turn into resentment, and he might do something to harm Harry's reputation, thus betraying him.