Naruto's character would develop in many ways. Firstly, he'd have to face the stigma associated with being the Demon of the Mist. This could make him more determined to prove himself. He'd also learn about the history and secrets of the Mist, which would add to his knowledge and wisdom. His relationships would change too. Some might fear him, while others might see him as a chance for the Mist to change. Overall, he'd grow into a more nuanced and adaptable character.
If Naruto is the next Demon of the Mist in fanfiction, his character development would be a journey of self - discovery. He'd start off confused and perhaps a bit scared of his new identity. But as he delved deeper into the role, he'd learn about the power and responsibility that came with it. He might have to suppress some of his more exuberant traits to fit in with the Mist's more somber culture. His understanding of good and evil would also be tested. He could find ways to use the power of the Demon of the Mist for good, like protecting the weak in the Mist village and reforming its more brutal traditions.
In this fanfiction scenario, Naruto might have to adapt to the harsh and secretive ways of the Mist. He could use his new status to bring about change. Maybe he'd combine his own ninja way with the Mist's traditions, like using his shadow clones in unique Mist - style assassinations. He could also face a lot of internal conflict as the Demon of the Mist is often associated with darker deeds, while Naruto is a hero at heart.
Naruto's character would likely become more withdrawn at first. The sudden change to being a frost demon would make him feel different from his friends. But as he learns to control his new powers, he'd become more confident. His sense of justice would still be there, but it might be tempered with the knowledge of his new, potentially dangerous abilities.
Naruto might become more conflicted. With the influence of the Demon God's power, he could question his own values. For example, the power might make him more aggressive in battle, which goes against his usual belief in protecting everyone. He would have to find a balance between using the new power and staying true to himself.
Naruto might become more empathetic. Loving a demon means seeing things from a different perspective. He would learn to understand the outcasts more, as the demon is likely an outcast in the ninja world. This would make him more inclusive in his actions towards others who are different.
His character could become more stealthy. Since the Hidden Mist Village is known for its assassins, Naruto might pick up some of their skills and become more proficient in covert operations. This would add a new dimension to his fighting style.
Naruto could develop a more complex view of good and evil. In Sabertooth, things might not be as black and white as in his ninja world. He'd see that some members of the guild have a rough exterior but good hearts. This would make him question his previous notions of what makes a person 'good' or 'bad'. His character would also grow in terms of his magical abilities, as he'd learn new spells and techniques in this new setting.
His character would be more feral. He'd be more in tune with his instincts like the demon foxes. So he might be quicker to anger and more impulsive in his actions.
If Naruto becomes the Demon of the Mist in fanfiction, he might start to lose his sense of self. The title 'Demon of the Mist' implies a certain level of malevolence and mystery that is quite different from Naruto's usual sunny disposition. His ninja way would be severely tested as he tries to balance the new, darker identity with his old values. Maybe he'd be forced to fight against his former allies who see him as a threat now. Also, his appearance could change to something more in line with the image of a demon, which would be a shock to those who know him well.
One interesting plotline could be the exploration of the Demon of the Mist's backstory. Maybe it reveals how the demon came to be associated with the Mist Village in the first place. Another could be a story where a character has to face the inner demons inspired by the Demon of the Mist and overcome them through self - discovery and growth.
His relationships could change. His friends might be in awe of his new form, and some might be a bit intimidated at first. But as they see Naruto is still the same person deep down, they would support him. Enemies, on the other hand, would view him as an even greater threat, which would put more pressure on Naruto to protect those he cares about. He would also have to learn to control his new power so as not to accidentally harm his friends with the wings.