Obviously, Naruto and Orochimaru are the main characters. Naruto is the protagonist who is going to Orochimaru, and Orochimaru is the one Naruto is going to see.
Another possible main character could be Sakura. She might be worried about Naruto going to Orochimaru and try to stop him. Or, she could follow Naruto secretly, and then there would be a whole new set of interactions when they are in Orochimaru's territory. Her medical skills could also come in handy if Naruto gets injured during the encounter with Orochimaru.
The main character is Naruto. In the past, he will surely encounter his parents. Minato and Kushina would be important characters in this fanfic. Also, Jiraiya, his mentor, would be a key figure. Since Naruto is going to the past, he might seek Jiraiya's guidance even more. And characters like Orochimaru would also be part of the story as Naruto may try to stop Orochimaru's evil plans earlier.
Obviously, Koyuki herself would be a main character to interact with Naruto. She might show him around her place, introduce him to her people. Also, any of her loyal guards or advisors could interact with Naruto. They could be initially suspicious of him but then grow to trust him as they see his good intentions.
In 'Naruto second Orochimaru fanfiction', Orochimaru is the central figure. Alongside him, Kabuto might be a prominent character as he was Orochimaru's right - hand man. Another character that could play a major role is Sakura. Since she is part of Team 7 with Naruto and Sasuke, and any threat or plot involving Orochimaru would likely draw her in. Also, there could be new characters created specifically for this fanfiction that interact closely with Orochimaru.
Well, Naruto is a given. Then there could be other minor gods or spirits in Kami's realm that Naruto meets. Maybe there's a guardian spirit of the ninja world that only appears when someone tries to reach Kami. Also, characters like Kakashi could be involved. He might have some knowledge about Kami that he imparts to Naruto before the journey, or he could even end up joining Naruto on his adventure to Kami.
It could be an evil version of Sasuke. In the original story, Sasuke had his dark moments, and in a very dark fanfic, he could be fully consumed by evil and oppose Naruto in a much more malevolent way.
Obviously Naruto would be one of the main characters. And from the Nova Corps, likely some of their leaders or important members like Rhomann Dey.
Obviously Naruto himself. Since he is the one going to the past, he would be the central character. His younger self would also be important as he would interact with his future self.
Well, obviously Naruto himself would be the main character. There could also be his friends like Sasuke and Sakura, who might be involved in the interactions between the two Narutos. Maybe they are confused at first by seeing two Narutos, but then they help them figure out what's going on.
It could still be Naruto himself. Since he's the central character in the original series, many fanfics would likely continue to build on his story.
The other main characters could include the Hokage of that time, like Hiruzen Sarutobi. Naruto might interact with him to try and influence the decisions of the village. Also, members of the Uchiha clan such as Itachi would play significant roles as Naruto could try to change their fates as well.