Natsu would likely become more reflective. Instead of always looking forward to the next adventure or fight, he would start to look back on his life. He might share stories of his past adventures with a new sense of nostalgia and wisdom, and his conversations with his friends would become deeper and more meaningful.
His impulsiveness might decrease. As an old man, Natsu would have more life experiences. He might think twice before jumping into a fight. For example, he would consider the consequences for his friends and the guild.
Maybe it's a result of some time - magic experiment gone wrong. Natsu accidentally gets caught in a time - distortion spell. This causes his body to age rapidly. In this state, he has to use his wits more than his brawn. He starts to see the world from a different perspective, and his relationships with his guild mates also change. For example, he becomes more of a mentor figure to the younger members, sharing his wisdom instead of leading the charge in battles.
His personality might become more cautious. Since he has a greater responsibility as a guardian, he can't be as reckless as he was sometimes in the original Naruto series.
His personality might become more withdrawn. As a lich, Naruto could be feared by others, and this might make him more hesitant to interact with people. He was always so outgoing before, but now he might keep to himself more.
Naruto may also become more solitary. Sparda often fought alone, and Naruto, influenced by this, might start to distance himself from his friends at first. However, he would still have that underlying loyalty and would eventually find a way to balance his new - found independence with his relationships.
If Tails becomes a robot in a fanfiction, his personality might become more logical and less emotional. Since robots are often programmed to be rational, he might start to analyze situations more coldly. For example, he might not be as quick to jump into a rescue mission out of pure compassion like he used to.
His confidence would skyrocket. Gilgamesh is extremely self - assured, and Naruto, if influenced by him, would become even more so. He'd be more outspoken about his own abilities and less likely to doubt himself in tough situations.
His friends might be initially scared. But then, they would likely be determined to help him. For example, Lucy might use her celestial magic to find a way to reverse the transformation. Their relationship would become more of a caretaker - patient one in the short term.
His confidence would likely soar. Knowing he has the speed of the Yellow Flash, Naruto would feel more capable in battle and in general. He might be a bit more cocky at times, but still retain his kind heart. For example, he could show off his speed to his friends in a friendly, competitive way.
His sense of humor might change too. Maybe he would use his mutant powers in funny ways to make light of the situation. For example, if he has water - based powers, he could make little water sculptures of his friends or create water fountains in unexpected places just to make people laugh. This would be a new side of his personality that combines his mutant abilities with his fun - loving nature.
Percy's personality would likely shift in several ways. He'd be more cautious in his actions as his decisions would have far - reaching consequences. His sense of humor might change too. Instead of the light - hearted quips he used to make, he might use more sarcasm or dry humor befitting a god. And his loyalty, which was always a big part of his character, might be tested as he has to balance his new divine duties with his relationships with his old friends. He may become more calculating in how he shows his loyalty, making sure it doesn't conflict with his new role as a god.