In this fanfiction, perhaps there's a power struggle. Naruto, being a powerful ninja, could be seen as a threat to the RWBY world's power balance. So, the RWBY characters, out of fear or a sense of self - preservation, betray him. They might think he'll disrupt their way of life or take over their territory.
If Naruto were betrayed in RWBY fanfiction, his values would be put to the test. Naruto believes in friendship and hard work. Being betrayed would make him question those beliefs. He might start to become more cynical, which would be a big change from his usual positive self. On the other hand, he could also use this as an opportunity to grow. He could learn about the different values in the RWBY world and try to find a new path. And perhaps, in the end, he would be able to forgive the person who betrayed him, but not without a long and difficult journey.
Obviously, Naruto would be one of the main characters. He's the one being betrayed. From RWBY, Ruby, Weiss, Blake, and Yang are likely to be involved. They are the central characters in RWBY, and if there's a betrayal of an outsider like Naruto, they would probably play a significant role in it.
Sakura would be furious. She's always been protective of Naruto in the original Naruto series. In the RWBY fanfiction, she would probably start her own investigation to find out who was behind the betrayal. She might even challenge the RWBY characters if she thought they were involved. Kakashi, on the other hand, would be more calm and collected. He would try to analyze the situation rationally and figure out the best way to help Naruto.
He could be betrayed by a false friend in the DxD world who spreads lies about him. For instance, telling others that Naruto has a hidden agenda to take over their realm.
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.
One possible scenario could be a character being betrayed by a teammate. For example, Ruby might be betrayed by Weiss if Weiss is lured to the dark side by some powerful and evil force. This could lead to intense battles and emotional turmoil for Ruby.
In a more complex scenario, Naruto could be influenced by an outside force. Let's say an enemy of Konoha uses mind - control or some form of manipulation on Naruto. They make him believe that the ANBU is the enemy of the village and that he needs to take them down. As he betrays the ANBU, he slowly starts to realize that he's been used, but by then he's already in too deep and has to find a way to fix the situation and regain the trust of his friends and the village.
In this fanfiction scenario, Naruto's betrayal by Akeno could be due to a misunderstanding of their goals. Maybe Akeno has been influenced by some dark forces that made her turn against Naruto. It could also be that there is some hidden information that led to this sudden change in their relationship.