Jiraiya. Even though he's dead in the original story, in fanfiction, his spirit could appear to Naruto during the fight. He could offer guidance or even transfer some of his remaining power to Naruto to help him defeat Pain.
Konan could be really interesting. In some fanfictions, she might have a different backstory or motivation that affects the Naruto vs Pain fight. For example, she could be secretly trying to stop Pain from going too far in his destruction of the Leaf Village.
Sakura. She could be given a more prominent role in the fanfiction. Maybe she has developed a new medical - ninjutsu technique that can counteract Pain's attacks. This would make her an important factor in the battle as she tries to keep Naruto alive and in fighting shape.
Jiraiya could be a great addition. Since he was Pain's teacher, his presence could add a lot of depth to the story. He could offer insights into Pain's past and also help Naruto understand Pain better.
Sasuke is often a popular character in these fanfictions. He might play a role in helping Naruto against Madara or have his own motives during their battle. Kakashi can also be a prominent figure, perhaps using his Sharingan in a crucial moment to assist Naruto. Another popular character could be Hashirama, as his connection to Madara is a key part of the original story and can be further explored in fanfiction.
Annabeth is always an interesting character. In fanfiction related to Percy's pain, she can be a source of support or sometimes a cause of additional stress for Percy. For example, if Percy is in pain because he fears losing her, it shows the depth of their relationship. Grover is also great. His loyalty to Percy can be explored in new ways. He might be trying to help Percy through his pain, using his nature - related powers or just his friendship.
The biggest difference is the creativity of the fanfiction writers. In the anime, the fight follows a set storyline. But in fanfiction, anything can happen. For example, Naruto could suddenly gain a new power that was never in the anime. It's like a whole new world of possibilities.
Naruto is, of course, very memorable. His determination to protect the village against Pain is always a central and inspiring part of any such fanfiction.
Naruto is definitely one of the most popular characters. Since he is the main protagonist in the original arc, he often takes center stage in fanfiction too. His growth, his new jutsu, and his interactions with Pain are frequently explored. Another popular character is Pain himself. His complex motives and powerful abilities make him an interesting subject. Writers often try to humanize him more in fanfiction.
It's hard to say for sure. If the fanfiction focuses on Hinata's growth, she could potentially defeat Pain through her own skills and some unexpected twists. But Naruto is also likely to be a major factor, so it could be a combined effort of both of them that overcomes Pain.
Maybe in the fanfiction, Naruto loses his chakra early on in the fight against Pain. But then he discovers a hidden power within himself that is not related to chakra, like a spiritual power from his Uzumaki heritage, which he uses to turn the tables on Pain.
Gaara could be an interesting addition too. Since he has his own complex backstory with power and isolation, he might see fem Naruto as a kindred spirit in a way. He could approach her to learn more about how she's handling the power and status, and perhaps they could form an alliance based on their unique positions in the world.
Sakura could be really interesting. She might be confused at first by Menma's presence, thinking he's some kind of impostor Naruto. But as she gets to know him, she could see his unique qualities and might even develop a friendly rivalry with him. Another interesting character could be Kakashi. He would be observing Naruto and Menma's interactions closely, maybe even giving them some special training to help them work together better.