Sakura might be the first to notice. She would approach Naruto gently and try to get him to talk about what's wrong. She'd offer him some home - made cake and sit with him, sharing stories of their past adventures to cheer him up.
Naruto would probably be quite affected. He is someone who has been through a lot of hardships in his own life, so a sad fanfiction about himself might bring back some of those difficult feelings. He could be sad for a while, but then he would likely try to shake it off and remember that it's just a story.
In fanfiction, Sasuke could be an ideal friend for Naruto. They have a complex relationship in the original story. In a fanfic scenario, they could put aside their differences earlier. They both have strong ninja skills and a desire to protect their village. Their friendship could be based on mutual respect for each other's abilities and a shared goal of making the Hidden Leaf Village stronger.
Sasuke is one of Naruto's best friends. Their relationship is complex and forms a central part of the story. They start as rivals but gradually develop a deep bond.
Eventually, Naruto could grow resentful. If the ignoring persists for a long time, he might start to think that Kakashi doesn't care about him at all. This could change Naruto's attitude towards Kakashi and might even make him more rebellious in his training or actions. However, deep down, he still probably hopes for Kakashi's recognition and approval.
Naruto would probably be extremely hurt. He has always considered Hinata a friend and perhaps more. Betrayal from her would be a huge blow to his trusting nature. He might initially be in denial, not wanting to believe that Hinata could do such a thing.
At first, Naruto would be completely shocked. He has always seen Hinata as a loyal friend and someone who has deep feelings for him. So the idea of her betrayal would be hard to fathom. He might try to deny it at first.
He would probably be very angry. Naruto has always been a free - spirited person, and being experimented on against his will would be a huge violation of his rights. He might initially try to resist physically, but as the situation gets more complex, he could start to plot his escape and revenge.
He would be filled with rage. Naruto has a strong sense of justice and the fact that someone took Hinata away from him would fuel his anger. He would vow to find and punish whoever was responsible. He might even become reckless in his pursuit of vengeance, endangering himself and those around him. His friends would have to step in to try and bring him back to his senses.