In some more creative fanfictions, even characters like Kakashi could be made to betray Naruto. Maybe he is put under a powerful genjutsu that makes him see Naruto as an enemy, or he is forced to choose between the village's interests (as misinterpreted by some) and Naruto, and unfortunately chooses the wrong side. However, this would be a more out - of - the - box scenario compared to the more common ones.
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.
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.
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.
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.
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.
Rias could be a character who betrays Issei in some fanfictions. If she were to face a difficult decision between protecting her family's reputation and standing by Issei, she might choose the former. Another possibility is Akeno. If she were to be influenced by some ancient evil power that made her turn on her friends, Issei could be a target.
Natsu could be a character who betrays Lucy in some fanfictions. Maybe he gets influenced by some dark magic that makes him turn on her. Gray is also a possibility. He might be jealous of Lucy's growing power or popularity in the guild and decides to betray her for his own gain.
Vegeta could be a likely character. In some fanfictions, his pride and envy of Goku's strength might lead him to betray Goku. He might think that by getting rid of Goku, he can become the strongest in the universe. Another one could be Frieza. Even if he pretends to be an ally in some alternate storylines, he could betray Goku at a crucial moment to regain his power and dominance. Also, a new and powerful character introduced in the fanfiction, who is envious of Goku's popularity and power, could easily be a betrayer.
Juvia. Although she is extremely loyal to Gray and Fairy Tail in the original series, in a fanfiction, if she were to misinterpret Gray's actions or words as a rejection of her love, she could become desperate. An enemy could take advantage of her vulnerability and persuade her that Fairy Tail is the reason for her unhappiness. Then she might turn against the guild.
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.