He might become more withdrawn. Without his happiness, Naruto would lose his usual exuberance. He would probably start to isolate himself from others, as he no longer has that positive energy to share. For example, he might stop going to his favorite ramen place and avoid his friends.
In the fanfic, Naruto's character could also become more introverted as he copes with the loss of his arm. He might need time alone to come to terms with his new reality. However, this solitude could also give him time to reflect on his life and his ninja path. He may emerge from this period with a new sense of purpose, perhaps focusing more on non - combat aspects of being a ninja, like diplomacy or teaching. This would be a significant shift from his previous more action - oriented self.
His character would take a dark turn. He'd likely become more aggressive. Without his humanity, Naruto may not show mercy or compassion in battles.
Naruto could become bitter. Losing trust, especially in those he once held dear, can make him view the world in a more negative light. He may no longer believe in the good of people so easily and might be more cynical. For example, he might not be as quick to offer help or friendship as he used to be.
Often, it starts with the event that causes Naruto to lose his eye. It could be a battle with a powerful enemy or an accident. Then, Naruto has to deal with the immediate aftermath, like the pain and shock.
One possible reason could be the loss of his friends. Naruto deeply values his relationships, and if something were to happen to his friends like Sasuke leaving again or getting seriously injured, it would shatter his happiness. Another reason might be if he fails to protect the village. Naruto has always seen himself as the protector of Konoha, so if he fails in this duty, his sense of purpose and happiness would be lost.
Maybe he starts to train with one - armed techniques. He could learn from other one - armed ninjas if there are any in the fanfiction world. His friends would also play a big role. They might encourage him to keep going or help him in his training.
The story could develop with Naruto being confused and making mistakes that he wouldn't normally make. His friends would try to help him, but he might push them away because he doesn't understand who they are to him anymore.
The storyline might start with Naruto being put in a really difficult situation that causes him to lose his mind. Then, the female Kyuubi could be introduced as a key factor in his recovery. Maybe she has some special ability or knowledge that can help him get better.
He might become more confident. Without the mask, he no longer has something to hide behind, and this new - found openness could boost his self - assurance.
His view of the world expands. While in the village, his perspective was somewhat limited. Now, traveling with his inheritance, he meets different people and cultures. This exposure broadens his understanding and makes him more empathetic. He might start to question some of the long - held beliefs in the ninja world. He could also become more secretive about his inheritance as he realizes its potential value and the danger it can bring.