How would Naruto's character change if he becomes a gangster in a fanfic?

He would become more cynical. Instead of his usual optimistic self, he'd view the world in a darker light. His trust in people would be severely damaged.

He'd become more responsible for the whole world perhaps. Since the Avatar has to keep balance everywhere.

With his friends in Konoha, like Sakura and Kakashi, there would be a major shift. Sakura might initially be conflicted between her loyalty to the village and her friendship with Naruto. Kakashi, being a more experienced shinobi, might understand Naruto's reasons better but still be bound by his duties to the village. Overall, Naruto's departure as a nukenin would create a lot of emotional turmoil among his relationships.

His personality would become more reserved. As an assassin, he couldn't be as open and friendly as he used to be. He'd have to keep his emotions in check to avoid being detected.

Naruto's personality would become more brooding. He would be less impulsive and more calculating, like Batman. He might not be as openly friendly as he was before, keeping his thoughts and plans more to himself.

If Naruto becomes cold, his relationship with Hinata would be severely affected. Hinata, who loves him for his warm heart, might be at a loss. Naruto might start to push her away, not allowing her to get close. His friends from Team 7 might try to bring back the old Naruto, but his coldness would create a rift between them.

Naruto's character would likely become more patient. Dealing with Pokemon, which are different from his usual ninja foes, would require a different approach. He would have to understand their behaviors and needs, which would make him more patient and empathetic. Also, his leadership skills from the Naruto series would be enhanced as he might lead groups of Pokemon trainers in this fanfiction world.

Naruto's character would gain a new sense of responsibility. As a Green Lantern, he'd have to abide by the Corps' rules and uphold justice on a much larger scale. This might make him more serious at times, but his core of being a fun - loving and determined person would still remain. He'd also learn to work in a team with other Green Lanterns, which could be different from his experience with his ninja teams as they come from diverse backgrounds.

Naruto's normally sunny and optimistic personality would likely become darker. He might be more brooding and less quick to trust. His sense of humor would probably take on a more sardonic edge as the power of Jack of Blades seeps into him.

With his friends in Konoha, they might initially be shocked by his new appearance. But then, they would get used to it. Naruto's popularity among the villagers could skyrocket. People who once shunned him might now be more accepting. This could also change his relationship with Iruka sensei. Iruka would be proud of how Naruto has grown not just in terms of power but also in his appearance, and their bond could become even stronger as Naruto becomes a more respected figure in the village.