There's a possibility that one of Naruto's friends or allies gives him the mask. For instance, Kakashi might give it to him. Kakashi is known for his own use of masks and could see Naruto's need for something similar. He could have acquired the mask during one of his solo missions and thought Naruto could make better use of it. Or perhaps it was given to Naruto by a new character introduced in the fanfiction, someone with a mysterious background who has their own motives for giving Naruto the mask.
In the 'naruto has a mask fanfiction', it might be that Naruto finds the mask himself. He could stumble upon it during one of his adventures, perhaps in an ancient ruin or a hidden cave. The mask could be enchanted or have some special property that draws Naruto to it, and he decides to keep it and figure out its secrets on his own.
The mask in the 'naruto has a mask fanfiction' might be a symbol of mystery. It could hide Naruto's true identity or feelings. Maybe he uses it to conceal a new power he has acquired and doesn't want his enemies to know about right away.
His enemies, too. They might realize that Naruto is not as straightforward as they thought. If he shows a more cunning side by dropping the mask, they would have to adjust their battle strategies. For example, Orochimaru might find Naruto a more formidable opponent than he initially believed.
His friends would be affected. For example, Sasuke might see Naruto in a new light and realize that they are more similar than he thought, both having their own inner struggles. Sakura would also be more empathetic towards Naruto and might try to be a better friend, offering more emotional support.
It could be Sasuke. He might be wearing a mask for some mysterious reason, like hiding his new powers or a new allegiance. Maybe he's on a secret mission that even Naruto doesn't know about at first.
Another possibility is that it could be a new character created specifically for the fanfiction. A character who wears a mask for some secretive purpose, and when the mask breaks, it sets off a chain of events that are unique to the fan - created storyline.
Obviously Naruto is one of the main characters. And the owner or creator of the mask could also be a main character, whether it's a new character introduced in the fanfiction or an existing character with a new role related to the mask.
Tobi is also a very popular character in 'naruto mask fanfiction'. Since he already wears a mask in the original Naruto series, fanfiction writers can expand on his identity, motives, and the true power of his mask. They might create scenarios where his mask has even more secrets than what was shown in the anime or manga.
Maybe it was Jiraiya. Jiraiya was a very important figure in Naruto's life. He could have given Naruto the mask as part of a training exercise. He might have thought that wearing a mask would force Naruto to rely more on his intelligence rather than his brute strength in certain situations.
Kakashi could be another option. He is a highly experienced and intelligent ninja. If he came across the skull mask, he would approach it with caution. He might study it first to understand its origin and power before deciding whether or not to use it. His calm and collected nature would be a great contrast to the potentially wild power of the mask.
It might give Naruto new types of jutsus. For example, if the Kitsune have the ability to create illusions, the mask could grant Naruto the power to use more advanced genjutsu. He could use these illusions not only in battle but also to deceive his enemies in strategic ways outside of combat.