Obviously, Naruto himself would be the central character. But also, his parents, Minato and Kushina, could play crucial roles. Minato could be there to mentor Naruto from a young age, passing on his powerful jutsu and strategic thinking. Kushina could teach Naruto about the power of the Uzumaki clan's chakra and their unique sealing techniques.
One idea could be that Naruto's early birth sets off a chain of events where he is trained in secret by an unexpected ally. Maybe a wandering shinobi who sees his potential and wants to keep him away from the prying eyes of the village elders. This could lead to Naruto having a different set of skills and a more mysterious reputation when he finally enters the Ninja Academy.
The main character, the first male born, might have a rival who is also from a powerful clan. This rival, let's call him Ren, could constantly challenge him. Hinata could be another important character, providing emotional support to the main character. She might see his potential and encourage him when he feels down. And of course, Sasuke could make an appearance, being a source of inspiration for the first male born in terms of power and determination.
Iruka could still be an important ally. Even though Naruto's situation is different, Iruka would probably be drawn to his positive attitude. And the Third Hokage would also be on his side. He was always looking out for Naruto in the original story, and that would likely continue. Plus, any of the ninja who were friends with Minato would probably be friendly towards Naruto as well.
The relationship with the village elders would be complex. They might have seen him as both a great hope and a potential liability. His relationship with the other Jinchuriki could also be different. If he was known earlier as a Jinchuriki, he might have been in contact with others, like Gaara, earlier. And his relationship with Sasuke would have an added layer. Maybe Sasuke's family would have seen Naruto in a different light, knowing he was born before the war and had some sort of special significance.
Obviously Naruto himself is a very popular character in this type of fanfiction. Since he's older, readers are interested in seeing how he has changed and grown. Sakura could also be popular. Maybe as an older and more experienced kunoichi, she has developed new medical ninjutsu or combat skills. Another popular character might be Kakashi. He could be in a more advisory role to Naruto, and his wisdom and experience would still be valuable.
The dogs themselves would be very important characters. There could be a lead dog, like a wise and powerful alpha dog that Naruto forms a close connection with. Maybe Akamaru could also play a role, since he is a dog in the Naruto universe already and could be involved in some way with Naruto's new contract.
Another option is a group of ninjas who are disillusioned with the current state of the Naruto world and the canon. They believe that by using the 'canon busters' concept, they can create a new and better world order. However, their methods are extreme and they end up becoming the main antagonists as they go against the main characters who want to preserve the essence of their world while also adapting to the new situation.
In this 'naruto after pain invasion fanfiction', Naruto is central, of course. But characters like Jiraiya could be important even though he's dead. His teachings and memories could still influence Naruto's decisions. Iruka, Naruto's first teacher, might have a role in helping Naruto deal with his emotional state. Gaara, as Naruto's friend and the Kazekage, could come to Konoha to offer assistance and strengthen the bond between their two villages. And then there are the other members of Naruto's team, like Kiba and Shino, who could be involved in various tasks related to the post - Pain invasion situation in Konoha.
The Uzumaki clan elders, even if they are not named in the original series. They could be important in a fanfiction. They could hold the keys to the clan's most guarded secrets. For example, they might know about a special Uzumaki training ground or a prophecy that involves Naruto and the future of the Uzumaki clan.
Well, in a fanfiction where Naruto is born before the Third Shinobi War, it could mean that the prophecy about him might have been known earlier. This could have led to different training methods for him. Maybe the Sannin would have been more involved in his upbringing. Jiraiya could have started training him much earlier, and Tsunade might have been more focused on his physical well - being. Orochimaru, on the other hand, might have seen him as an earlier opportunity for power and tried to manipulate him much sooner. All these changes would have a huge impact on the overall Naruto storyline.