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.
One implication could be that it strengthens the Uzumaki presence in the story. With Naruto already being a prominent Uzumaki, Karin's addition means there are more Uzumaki characters around. It could also mean that they might have some special abilities or powers related to their clan that could play a role in future events. For example, their strong life force which is characteristic of the Uzumaki clan.
In many fanfictions, Naruto's reaction is a mix of shock and determination. The shock comes from finally learning about his clan, and the determination is to uphold the Uzumaki name. He could start wearing something that represents the Uzumaki clan, like a symbol on his clothing. He also may become more protective of the Uzumaki secrets and legacies he discovers.
He is probably very excited. Naruto has always been eager to learn new things and become stronger. Finding an Uzumaki scroll means he has the chance to discover something unique about his own clan, so he would be filled with anticipation.
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.
Sasuke might be initially shocked. He has always seen Naruto as a rival and suddenly finding out Naruto is a girl could completely change his perception. He may start to view Naruto in a more protective way, as his traditional views of gender roles might make him feel the need to safeguard her now that he knows she's female.
Well, Kakashi is a complex character. When he discovers Naruto is abused in the fanfiction, he may first feel a deep sense of guilt for not noticing earlier. He'd then use his skills and influence in the village. He could start by reporting it to the proper authorities in Konoha, like the Hokage. And at the same time, he'd be there for Naruto, providing emotional support, talking to him, and helping him through the trauma. Kakashi has his own past traumas, so he'd understand what Naruto is going through on some level.
She would likely be devastated. Kushina has a deep connection to Naruto as his mother. She might first go into denial, unable to accept that her son is gone. Then, she would probably be filled with intense grief and anger. She could potentially lash out at those she blames for his death or just break down completely, mourning all the things Naruto could have been and all the time they missed when he was growing up.
Some in the Naruto community might be really excited about it. They like to see new and different pairings. It gives a fresh take on the Naruto universe. Fans who enjoy exploring different character relationships would be all for it.
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.
A fanfiction could start with Naruto and Karin being forced to team up due to a prophecy. At first, they clash because of their different personalities. Naruto is all about friendship and believing in people, while Karin is more cautious and self - centered. But as they face stronger enemies, they start to see the value in each other.