Naruto characters' reactions would vary. Shikamaru would likely find the whole situation troublesome. He doesn't like getting involved in things that are too complicated. But Naruto would approach the archangel with his usual enthusiasm. He'd want to be friends and learn about the archangel's world. Hinata would be shy but also intrigued. She might see the archangel as a very noble and powerful being. And Kiba with Akamaru would be a bit more aggressive in their approach, ready to defend themselves if needed but also eager to know more about this new entity.
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 a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
The friends' reactions would depend on their own personalities and relationships with Issei and Rias. For instance, Rossweisse, being a Valkyrie, might have a more cautious approach. She could be worried about the potential disruption to the balance between the different supernatural groups. Meanwhile, Irina, with her strong beliefs, might struggle initially to accept the situation but eventually come around due to her friendship with Issei and Rias. And of course, all of their friends would likely rally around them to help with the challenges that come with a pregnancy in the High School DxD world.
Asta could be brought into the High School DxD fanfic as a result of a multiverse - related event. In this case, different universes are colliding or being connected in some way. His presence would disrupt the power balance as his anti - magic is something that no one in the High School DxD world has really encountered before. He could end up training with the devils to learn more about their magic while also teaching them about the concept of anti - magic.
One way could be through the power system. The EMS (Eternal Mangekyou Sharingan) from Naruto could be a unique power that a character in the High School DxD world acquires. Maybe a transfer student from a hidden ninja village arrives at the high school and has to keep their EMS a secret while dealing with the devils and angels in the school. This could lead to some interesting battles and interactions as the powers of the EMS, like the ability to use powerful jutsus, contrast with the magic and abilities of the High School DxD characters.