Naruto might be angry at first. He is not one to take betrayal lightly.
Naruto would probably feel a mix of emotions. Hurt, because he thought he could trust Rias. Angry, as it's a betrayal. But he would also be strategic. He would use his skills as a ninja, like his stealth and combat abilities. He might try to gather information secretly about Rias' actions. He could also reach out to his friends in the Naruto world for advice or help, while at the same time trying to keep a low profile in the world where Rias is, in case she has more plans against him.
He could initially be in disbelief. But then, his determination would kick in. He would likely try to figure out a way to make things right or get revenge, depending on how the story is written. Naruto has a strong sense of justice, so he may also be interested in finding out the root cause of the betrayal to prevent it from happening to others.
Naruto might initially feel disbelief. He's used to seeing the good in people, so Akeno's betrayal would be a huge shock. He could go into a state of self - doubt, wondering if he misjudged her all along.
Naruto would likely be extremely hurt at first. He values loyalty highly, and this betrayal would be a big blow to him. He might go into a state of shock and disbelief.
Naruto's reaction could be a mix of sadness and resolve. He has a big heart, so the betrayal would cut deep. But he also has the will of fire within him. He could use this as fuel to rise above the situation. He might seek out new allies in the High School DxD world, like Issei and his friends (if they weren't part of the betrayal). Together, they could work towards uncovering the truth behind the betrayal and taking on any threats that might be related to it.
In this kind of fanfiction, it might happen because of a misunderstanding. Maybe Rias is misled by false information about Naruto. For example, some villains could fabricate lies about Naruto's intentions or actions, causing Rias to believe he's a threat and thus betray him.
He would be heartbroken. Naruto has a big heart and has always trusted his friends. This betrayal would be a huge shock to him.
Naruto would be deeply hurt. He has grown up with so little love and acceptance in the village, and Hiruzen was one of the few people he thought cared for him. His reaction could involve a lot of self - reflection. He might question his own worth and what he has done wrong. But ultimately, his positive nature would take over. He would try to understand Hiruzen's reasons, even if he didn't agree with them. And he would strive to find a way to reconcile and make Hiruzen see that he is not a threat.
At first, Naruto might try to reason with the Western side. He would want to understand why they betrayed him. He believes in the good in people, so he would hope that there was some misunderstanding. However, if they continued to be hostile, Naruto would become determined. He would gather his own allies, like his friends from Konoha, and form a resistance against the Western forces. He would use his powerful jutsus and his unwavering spirit to take on those who wronged him.
In fanfiction, such a betrayal could occur due to power struggles. Maybe Rias sees Naruto as a threat to her position in the Highschool DxD world. She might fear his growing power and influence, which could overshadow her own. So, in order to maintain her status, she betrays him.
He could be initially shocked and heartbroken. Naruto has always been loyal to Konoha, so such a betrayal would hit him hard. He might go into a state of disbelief for a while.
He would be devastated. Naruto has always considered Konoha his home and the people there his family. At first, he might not believe it, thinking it's all a misunderstanding. But as the truth sinks in, he would probably be filled with anger and sadness. He might leave Konoha, but his sense of justice wouldn't let him just forget about the betrayal. He could seek revenge on those who wronged him, or he might try to prove his innocence from afar.