Naruto would likely be deeply hurt. He is a character who values friendship and loyalty highly. Being betrayed by someone he might have considered an ally like Rias would be a huge blow to his emotional state.
In this kind of fanfiction, Naruto might face a lot of challenges. Maybe Rias, for some reason, turns against Naruto. Naruto could find himself isolated and having to fight against Rias' forces. He would probably rely on his ninja skills and his strong will to survive.
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.
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.
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 would likely become very distrustful. After being betrayed by Rias, he would find it hard to trust anyone else in the Highschool DxD world. This could isolate him and make his journey much more difficult as he would be on his own without allies.
Well, in fanfiction, the author often likes to create complex and unexpected plot twists. It could be that Naruto's loyalty to his own values and beliefs clashes with the goals and ethics of Rias, Sona, and Ravel. For instance, Naruto may be against certain practices that they engage in, like how they handle power or deal with other groups. This ideological difference could lead to their betrayal. Also, there could be a love triangle or some sort of relationship drama involved. If Naruto is involved with someone they all like or if there are misunderstandings in relationships, it could spark the betrayal.
They would likely start with some mutual curiosity. Naruto would be intrigued by Rias's devilish appearance and powers, while Rias would be interested in Naruto's ninja abilities. They might exchange stories about their respective worlds, with Naruto talking about his village and his friends, and Rias sharing about the devil society. Interaction could also involve sparring sessions to test each other's strength.
One possible fanfiction idea could be that Rias' sick brother is a key factor in a power struggle within their supernatural world. Naruto, being an outsider with his own set of skills and values, could get involved. Maybe he forms an alliance with Rias to protect her brother from those who would use his illness to their advantage.