Their relationship would change in many ways. Firstly, Naruto would be more involved in Gaara's life. He would probably train with Gaara more often, teaching him new techniques. Secondly, Naruto would be a source of emotional support for Gaara. Given Naruto's positive attitude, he could help Gaara overcome his past traumas more effectively. And finally, they would have a stronger sense of loyalty towards each other. In battles, Naruto would not hesitate to put himself in harm's way to protect Gaara, and Gaara would do the same for Naruto, much like a real - life brotherly relationship.
His sense of family would be stronger. Naruto has always longed for family, and having an older brother would fulfill that to some extent. This could make him more empathetic towards others who are also missing family members. He might also be more cautious in battles, as he wouldn't want to let his brother down.
If Naruto were Itachi's older brother in fanfiction, the story would be drastically different. For one, Naruto's positive influence might have changed Itachi's path. Itachi was forced into many difficult decisions in the original story. With Naruto as an older brother, he could have had more support and might not have made the same choices regarding the Uchiha massacre. Naruto's ideals of friendship and never giving up could have rubbed off on Itachi, leading to a more optimistic Itachi. Also, the relationship between Konoha and the Uchiha might have been better managed with Naruto's presence as he is known for his ability to bring people together.
Well, if Naruto were Kushina's older brother, the whole story of Naruto would be turned on its head. The story would start with Naruto already having a family connection to the Uzumaki clan. His training and development would likely be different as he would have grown up with Kushina. He might have learned some unique Uzumaki techniques from their family. His relationship with the other characters would also change. For instance, his relationship with Jiraiya might be different as Jiraiya might also see Naruto in a different light due to his relation to Kushina. And when it comes to the big events in the Naruto universe, Naruto would be in a position to directly influence Kushina's actions, which could change the course of many events.
Their relationship could be a major driving force in the plot. Naruto could act as a mentor to Hashirama, guiding him in his decisions regarding the village. This would likely result in a much more developed and perhaps more just village from the start.
Their relationship could also have a bit of a fun side. Naruto might play pranks on Serena sometimes, but in a harmless way. And Serena would get back at him in her own cute ways. They would have deep conversations too, where Naruto shares his wisdom from his ninja experiences and Serena shares her thoughts about the world around them.
His abilities might be enhanced by the demonic powers associated with Sirzechs' world. For example, his Rasengan could take on a more demonic form, with added destructive power. Maybe he could also gain the ability to teleport short distances like some demons in Sirzechs' world, which would give him an edge in combat.
At Hogwarts, Harry would have someone to look up to within his family. This could boost his self - esteem. His older brother could be popular, and through that, Harry might gain more friends. Also, they could team up in Quidditch, making Gryffindor's team even stronger.
At first, their relationship might be a bit guarded. Gaara, being used to isolation, might be hesitant to open up to Naruto. But as they face challenges together, like fighting a common enemy, they would start to respect each other's strength. Naruto's positive attitude would gradually break down Gaara's walls, and they would become friends.
Naruto might become more confident earlier. With a brother by his side, he would have someone to rely on and confide in from the start. This could make him less of an outcast in his own mind and more willing to open up to others in the village.
It would be more nurturing. Kurenai could cook for Naruto, like making him some of her special ninja - style meals. And she'd be there to listen to his problems, whether it's about his love life or his struggles with mastering new jutsu. Their relationship would be based on mutual trust and respect, just like a real sister - brother relationship.