In a fanfiction where Naruto has a secret Rinnegan, who would be the first to discover it?

Well, it could make Naruto extremely powerful right from the start. With the Rinnegan, he might be able to master all types of jutsu more quickly. He could also have an edge in battles against powerful foes like Madara or Kaguya. Maybe he'd be able to use the six paths techniques with more ease and less training.

His relationship with Sasuke would likely become more complex. Sasuke might be both jealous and intrigued by Naruto's new powers. Naruto would have to prove that he still values their friendship despite his increased strength. With Sakura, she might see Naruto in a new light, perhaps with a bit more admiration as he now has such rare and powerful abilities.

In a fanfiction where Naruto secretly has the Rinnegan, he would become an even more formidable force. His combat abilities would skyrocket. He could potentially use the powers of the Rinnegan, like the Six Paths techniques, in addition to his existing Sage Mode and Nine - Tails chakra. This would make him a major threat to any enemy. He might be able to travel between dimensions more easily and have better control over life and death, just like Madara and Sasuke did with their Rinnegan.

Edward is a likely candidate. Since he is so attuned to Bella, he might pick up on some strange behavior or scent that leads him to suspect she is hiding something.

His relationship with Sasuke would likely be very different. Since Sasuke has also had experiences with the Rinnegan, they might now have a new level of understanding between them. They could train together to master the powers of the Rinnegan or compare notes on how to use its abilities.

Naruto's character would likely become more confident. Having the Rinnegan, a powerful and rare dojutsu, would give him an edge in combat and in understanding the world around him. His skills in sealing would also make him more versatile. He might become more of a thinker, as he has to combine the two abilities effectively. For example, when facing a difficult situation, he would first analyze it with the Rinnegan and then come up with a sealing solution.

Naruto would likely become more introspective. The power of the Tomoe Rinnegan is not just physical but also mental. He might start to question his place in the world more deeply, as this new power sets him apart from others. He could become more cautious in his actions as he realizes the potential destruction his new power could cause if misused.

The story could develop with Konoha facing various threats without Naruto's help. They might initially struggle and have to rely on their remaining ninja forces. This could lead to the development of new characters or the growth of existing ones as they are forced to step up. Meanwhile, Naruto could be off on his own journey, perhaps exploring the true nature of the Rinnegan's power or trying to create a new world order without Konoha's influence.

In fanfiction, Naruto might keep the Rinnegan a secret by avoiding using its powers in public. He could also use his shadow clones to create distractions when he feels like the Rinnegan is about to be revealed. Plus, he could tell only a very few trusted people, like Kakashi or Iruka, and ask them to keep it a secret for him.

Another possibility is that it's a result of some strange chakra experiment gone wrong (or right). Scientists in the Naruto universe are always dabbling in chakra research. Maybe an experiment meant to enhance her natural abilities instead gives her the Rinnegan, and she then has to learn to control it quickly to avoid being consumed by its power.