It might be that the truth is about his own origin and the prophecy associated with him. He could discover that the village had been manipulating him all along based on this prophecy. For example, they might have been using him as a weapon without his full knowledge, and once he finds out, he decides to leave to find his own path away from their control.
One possible consequence could be a shift in the power dynamics within the village he leaves. His departure might leave a void that could be filled by either positive or negative forces. Also, his friends would likely be deeply affected, feeling a sense of loss and perhaps even betrayal.
The truth could also be related to his feelings. Perhaps he's in love with Hermione more deeply than she realizes, and there are some misunderstandings that get cleared up when she finds out. In certain fanfictions, Ron might have been sacrificing his own opportunities for Hermione's sake, like giving up a chance to study with a great wizard so she could go instead.
She might be overjoyed. After all, Naruto is her son. She could immediately rush to find him and embrace him tightly, tears of happiness streaming down her face.
However, if the truth is something more complicated or even a bit negative, like Ron has been lying about his family's financial situation to seem more worthy of her, it could cause a rift. She might feel betrayed at first, but depending on how Ron explains and if he's truly sorry, they could work through it.
In many fanfictions where Naruto discovers he has a sister, it often changes the entire dynamic of his story. It could give him a new sense of family and belonging that he lacked growing up. Maybe this sister was hidden away for some reason, like to protect her from Naruto's jinchuriki status bringing danger to her. This new relationship can also influence how Naruto approaches his ninja goals and his relationships with his friends.
When Naruto discovers Jiraiya's death in fanfiction, it's a moment of pure shock. He may immediately recall all the lessons Jiraiya taught him, from the Rasengan to the importance of never giving up. This news could fuel his determination to become stronger, not just for himself but also to avenge Jiraiya. He might also have flashbacks of their adventures together, which only intensifies his pain.
In such a fanfiction scenario, Naruto would likely be extremely hurt. He has deep feelings for Hinata, so finding out she cheats would shatter his trust. He might initially go into denial, not wanting to believe it. Then, he could become angry and might distance himself from Hinata. His friends would probably try to console him, but it would take a long time for him to get over the betrayal.
Naruto might initially be confused. He has grown up with a certain view of himself, and this new information about his heritage completely shatters that. He may struggle with his identity for a while, but as he digests the information, he could start to feel a sense of pride. He might think that now he has a greater purpose, something that ties him more closely to his family and the ninja world in general. He may also become more confident in his abilities, knowing that he comes from a great lineage.
Naruto's reaction could be complex. On one hand, he would be happy to find someone else from his clan. He has always been proud of his Uzumaki heritage despite not knowing much about it at first. But on the other hand, he might be a bit cautious. Karin has been associated with some not - so - good groups in the past. However, overall, his positive nature would probably lead him to approach Karin with an open mind and try to build a connection with her based on their shared Uzumaki roots.