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.
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.
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.
Some fanfictions portray Ophis as a misunderstood character. Despite her fearsome power, she may actually have good intentions deep down. For instance, she could be trying to protect something or someone in the Fairy Tail world but her methods are misconstrued by the guild members at first.
They could also have a mentor - mentee relationship in a way. Naruto, with his experience in combat and growth, could offer advice to Ophis on how to better control her power. Ophis, with her knowledge of the DxD world's magic and power systems, could teach Naruto some new skills or ways to enhance his chakra. This exchange of knowledge and experience would create a unique dynamic between them.