The devil could be a new, mysterious character created specifically for the fanfiction. It might be an ancient evil entity that has been lurking in the shadows of the Naruto world for ages.
Maybe it's a transformed version of one of Naruto's known enemies. For example, Orochimaru could have made some dark pact and become a devil - like figure that Naruto then has to deal with. This would add an interesting twist to the story as Naruto has a pre - existing relationship with Orochimaru.
It could be a new character created specifically for the fanfic. Maybe a powerful and malevolent spirit that has been lurking in the shadows of the Naruto world.
Well, perhaps Naruto makes the deal to save his friends. He might trade his future or part of his soul to the devil in this fanfiction. And then he has to struggle with the consequences, like dealing with the devil's constant demands or trying to find a way to break the deal without losing everything.
In such a fanfic, Naruto might gain extraordinary power. For example, the devil could offer him the ability to defeat any enemy instantly. But there would be a price, like sacrificing his relationships with his friends.
Madara Uchiha could also be a candidate. His view of the ninja world was very different from Naruto's. He might play the Devil's Advocate by trying to persuade Naruto that the cycle of hatred can only be broken by a more dictatorial approach, rather than Naruto's ideal of love and understanding. He could use his own experiences and the history of the Uchiha clan to make his arguments seem valid.
The village elders could be antagonists. They might see Naruto as a threat due to his devil - like origin and try to eliminate him. They fear that his powers could bring chaos to the village.
It could be a powerful devil who sees Naruto as a threat to his rule in the devil realm. Maybe this devil wants to use Naruto's power for his own evil purposes and Naruto has to resist.
It could be a powerful devil who has the ability to control minds. This devil could manipulate some of Naruto's friends and turn them against him, creating a lot of internal conflict within Konoha.
A new character could be introduced as the main villain. For example, a powerful demon from another realm who has the Devil Trigger ability and wants to conquer Naruto's world. This demon could have minions with similar powers and Naruto would have to figure out their weaknesses in order to defeat them.
He could eat the Pika Pika no Mi (Glint - Glint Fruit) which gives the power of light, allowing him to move at the speed of light and create powerful light - based attacks. And perhaps the Yami Yami no Mi (Darkness - Darkness Fruit) which can absorb other attacks and create a gravitational pull. Combining the powers of light and darkness would be quite a unique and powerful combination for Naruto in this fanfiction.
Well, Harry Potter himself would be a main character of course. Since it's about him dealing with the devil, he's at the center of the story. Then there would likely be some sort of devil - like figure. It could be a new dark entity created just for this fanfiction or it could be a darker interpretation of an existing character, like Voldemort being given more devilish traits.