Lucifuge could betray Naruto to the Akatsuki. The Akatsuki has its own goals and they might see Naruto as a threat. Lucifuge, if he has some sort of vendetta against Naruto or his village, could team up with the Akatsuki. He could provide them with information about Naruto's abilities, weaknesses, and the layout of the village, which would give the Akatsuki an edge in their plans to capture Naruto.
He could betray Naruto to Orochimaru. Orochimaru is always looking for powerful allies and new test subjects. Lucifuge might be lured by Orochimaru's promises of power and knowledge. Orochimaru could use Lucifuge to get closer to Naruto in order to steal the Nine - Tails' chakra or some other valuable asset.
In a Naruto Lucifuge betrayed fanfiction, the betrayal could be part of a much larger conspiracy. Lucifuge might be under the control of a powerful dark force. He betrays Naruto not out of his own will, but because he is being manipulated. This could lead to a journey for Naruto to not only stop Lucifuge but also to find a way to free him from the control of this dark force. Naruto would have to face his own feelings of trust and friendship being broken while also trying to save his friend from the clutches of evil.
Well, Naruto is obviously one of the main characters. He's the protagonist of the Naruto series, and in the fanfiction involving Lucifuge, he's likely to be at the center of the story, dealing with the new elements related to Lucifuge. And then there's Lucifuge itself, which is the new and mysterious element introduced into the Naruto universe in this fanfiction. It could be a character with its own motives and powers.
One interesting Naruto Lucifuge fanfiction could be about Naruto somehow getting in touch with Lucifuge's powers and how he uses them in the Naruto universe. Maybe he uses these new powers to face stronger enemies in a unique way.
It could be one of Naruto's former teammates from a mission. Maybe this teammate has some hidden motives or has been bribed by an enemy. They could spread false information about Naruto within the community, making it seem as if Naruto has betrayed them all during a crucial mission. This would cause the rest of the community to lose trust in Naruto.
It could be Zabuza. He might pretend to be on Naruto's side at first during the Wave events and then betray him when Naruto least expects it. Zabuza is a mercenary and his loyalty can be easily swayed.
Even Hinata could potentially be written as a betrayer in some really out - there fanfictions. Maybe in a crossover where there are mind - control elements, Hinata could be forced to turn against Naruto. But this would be a very different take on her character as she's usually portrayed as extremely loyal to Naruto.
The world in which Grayfia exists is full of magic and intrigue. Fanfiction writers can build on this world, adding their own details and plotlines. They can create stories that focus on her role in the grand scheme of things, or they can zoom in on her personal growth and development. All these elements combined make 'grayfia lucifuge fanfiction' very appealing.
There's a chance that some of his former teammates who have their own insecurities. For example, someone who always felt in Naruto's shadow might be the one to betray him in order to get more recognition for themselves.
It could be Sasuke. He might be jealous of Naruto's new power and feel overshadowed. So at Wave, he decides to betray Naruto to prove his own worth.
Sasuke could be a likely candidate. He's always been somewhat jealous of Naruto's growth and popularity. In the stress of the Forest of Death, he might let that jealousy get the better of him and betray Naruto. Maybe he thinks that by getting rid of Naruto, he can be the only strong one in the team.