I'm not entirely sure as it's a fanfiction, but Ophis is likely a unique character created for this particular story. Maybe she's a powerful witch or some sort of magical being.
Natsu could be a great character to pair with Ophis. His fiery and impulsive nature could contrast well with Ophis's more mysterious and powerful self. They could go on quests together where Natsu's brute strength and Ophis's magic complement each other.
One possible plotline could be Naruto, as a male, having some kind of special connection with Ophis. Maybe he discovers a power within himself that is related to Ophis' nature. For example, he could gain the ability to control a certain type of magic or energy similar to what Ophis has.
At first, Naruto might see Ophis as a powerful ally he needs to understand in order to protect his world. He would try to approach her with respect. Ophis, seeing Naruto's pure heart, would allow him closer. They would then go through various challenges together, like facing enemies that threaten both their existences. During these trials, they would rely on each other, and Naruto's positive attitude would start to melt Ophis' cold exterior. Their bond would strengthen over time until they realize they can't live without each other and decide to get married.
I'm not sure of a specific one off the top of my head. But you can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Naruto is male Ophis' in the search bar and you might find some interesting stories.
One possibility is that it could be a moment of great shock for Naruto. He might be initially flustered, his face turning red as he tries to figure out what's going on. Maybe Ophis is seeking comfort or has some ulterior motive in this rather intimate - for Naruto - position. He could stutter and try to ask her what she's doing.
He would probably feel embarrassment. Naruto is a bit of a blusher when it comes to unexpected physical contact, especially from a female. His cheeks would turn red and he would struggle to find words.
It's possible that there could be a fanfic with this premise. Fanfic writers often explore very creative and strange ideas. However, without doing a very extensive search in the vast world of fanfiction websites, it's hard to say for certain. There are so many different takes on existing fictional universes in fanfic, and this would be a very interesting and unexpected one.
It could show a friendly relationship. Maybe they have become comrades after some adventures together. Ophis feels comfortable enough around Naruto to sit on his lap casually, like how good friends would have a relaxed and trusting interaction.
One idea could be a story where Naruto is somehow transported to the DxD universe and meets Ophis. He could form an alliance with her, perhaps because they both have a sense of being outsiders in the new world. Naruto's unique skills and his never - give - up attitude could influence the events in the DxD world in interesting ways.