Well, one way it could work is if there was some sort of multiverse crossover. Maybe in a strange twist of magic, Naruto could be transported to the Fairy Tail universe and end up related to Mavis. It would be really interesting to see Naruto's ninja skills combined with the magic of Fairy Tail. He could teach the Fairy Tail guild some unique combat techniques.
In a fanfiction, the author has a lot of creative freedom. To make Naruto the brother of Mavis, the author could create a backstory where Naruto was born in a world that has elements of both the Naruto universe and the Fairy Tail universe. His upbringing could be influenced by both ninja training and Fairy Tail magic. His relationship with Mavis could be a driving force in the story, like they are on a quest to unite the two worlds or protect them from a common threat.
In a Naruto - Fairy Tail fanfiction where Naruto is a brother figure, he might bring a whole new dynamic. He could teach the Fairy Tail guild members some of his ninja training techniques. For example, how to better endure pain or how to focus their energy more effectively. Also, his backstory of being an outcast in his own village at first could make him relate to some of the Fairy Tail characters who have had difficult pasts.
In some fanfictions, he is a mentor - like figure. He teaches Mavis or other young wizards important lessons about magic, life, and the values of Fairy Tail. This could include things like the importance of friendship in magic, which is a central theme in the Fairy Tail series.
One way could be through a crossover of magic and ninjutsu. Maybe Mavis's brother could have a unique power that is similar to a ninja ability in Naruto. For example, if he has some kind of illusion - based magic, it could be related to the genjutsu in Naruto. Writers could create a scenario where he teams up with Naruto characters, sharing his Fairy Tail - like magic and learning from the ninjas at the same time.
In fanfiction, Mavis' brother can be portrayed in various ways. Some might depict him as a protective figure, always looking out for Mavis before her misfortunes. He could be a strong magic user, with unique abilities that are different from Mavis' own. For example, he might have a more combat - oriented magic compared to Mavis' strategic and leadership - related magic.
Naruto could be a protector. Just like he protected Konoha in his own world, he would protect Mavis and the Fairy Tail guild. His combat skills would be a great asset.
It could work by making Naruto a more mysterious and brooding character. In Fairy Tail, there are many characters with different personalities. An emotionless Naruto would stand out. He might be more focused on achieving his goals without being distracted by emotions. For example, he could be on a mission to find a way back to his own world and his lack of emotion helps him stay determined.
One possible plot could be that Naruto somehow gets transported to the Fairy Tail world and meets Mavis. Naruto, with his never - give - up attitude, could inspire Mavis in a new way. He might help her deal with some of the darker forces in the Fairy Tail universe using his ninja skills. For example, Naruto could use his shadow clone jutsu to confuse enemies while Mavis uses her Fairy Magic to attack from another angle. They could also exchange stories about their respective worlds and learn from each other's leadership styles, Naruto as a Hokage - in - training and Mavis as the founder of Fairy Tail.
He could be a catalyst for change. Maybe his arrival in the Naruto world disrupts the normal balance, forcing the characters to adapt to new ways of thinking. For example, his Fairy Tail - like values of friendship and loyalty could influence the Naruto characters to reevaluate their own relationships.
Naruto has shown incredible feats of strength, speed, and endurance. He has defeated some of the most powerful villains in his universe. But Mavis is no slouch either. Her Fairy Law is a very powerful spell. If we consider their growth potential, Naruto has the ability to constantly get stronger by unlocking new forms of his power, like Sage Mode and later the forms related to the tailed beasts. Mavis, on the other hand, has a deep knowledge of magic that could potentially allow her to counter any of Naruto's attacks. It really depends on the situation, the location, and how well they know each other's abilities.
One possible plot could be that there is some sort of misunderstanding or dark magic at play that causes Natsu and Mavis to fight. Natsu, being hot - headed, jumps into the fray without fully understanding the situation. Mavis, on the other hand, is trying to figure out a way to stop the fight without harming Natsu. As the battle progresses, they both start to realize that they are being manipulated and then team up to face the real enemy.