The main characters are Naruto and Bellemere for sure. But we might also see characters from Bellemere's past or her village. Maybe there's an old friend of hers who provides some crucial information to Naruto about where she's being held or who has captured her. This could add more depth to the story and help Naruto in his quest to save her.
In the 'naruto saves bellemere fanfiction', it could start with Bellemere being captured by a group of rogue ninjas. Naruto, being the hero he is, gets wind of this. He uses his rasengan to break through any barriers or defenses the rogues have set up. He then finds Bellemere tied up and frees her, and they make their escape together. Along the way, Naruto might share some of his dreams and beliefs with her, inspiring her as well.
There could be some of the older Uzushiogakure ninjas who survived the previous disasters. They would play an important role in guiding Naruto about the secrets of the village. And maybe some new characters could be introduced, like a young Uzushiogakure ninja who looks up to Naruto and helps him in his quest to save the village.
Obviously, Hermione and the Weasleys. Hermione is the one doing the saving, so she is the central character. The Weasleys as a whole family would also be important, including characters like Molly, Arthur, Ron, Ginny and the others.
Sasuke is very likely to appear. Since he and Naruto have a deep bond, he would surely come to Naruto's aid when it comes to saving his family. Also, Sakura, as Naruto's teammate and a powerful ninja in her own right, would be there. Kakashi could also make an appearance, providing guidance and wisdom to Naruto, just like he did when Naruto was younger.
One possible antagonist could be a rogue ninja from another village. They might target Tenten for her unique weapon - handling skills, thinking they can use her to create powerful weapons for themselves. Naruto would then have to face this ninja to save Tenten.
It could be some rogue ninja clans that are after Karin for her special abilities. They might see her as a tool to gain power or information. For instance, a clan that specializes in blood - related jutsu might want to use Karin's unique blood - sensing ability for their own evil purposes.
One potential antagonist could be a power - hungry shinobi from another village. They might view Hayate as a threat to their plans, maybe because Hayate is involved in some sort of peace - keeping mission between villages. So, they try to eliminate him. And Naruto, being the defender of his comrades, will not stand by and let that happen. There could also be a group of missing - ninjas who band together to take on Hayate, thinking he's an easy prey, but they underestimate Naruto's loyalty and strength.
In a 'naruto saves korra fanfiction', the villains could be a combination of different forces. There might be rogue benders who have turned to the dark side, using their bending abilities for evil purposes. These benders could be working with some mysterious outsiders who bring in new and unknown threats. Maybe there are ancient evils that have been awakened and they see Korra as a major obstacle. Naruto would then have to face these complex and dangerous foes to save Korra.
Maybe some new rogue ninja group that has a vendetta against the Uzumaki family. They see Boruto as a way to strike at Naruto, so they try to harm him, which then leads to Naruto having to save him.
Obviously, Naruto is one of the main characters. He is the one being saved. And then, members of the Akatsuki like Itachi, Kisame, or Pain would likely be main characters as well. Itachi, for instance, has his own complex motives and backstory that could play into the story of saving Naruto. Kisame is a powerful and loyal member of the Akatsuki who could be involved in the rescue mission. Pain, as the leader in some cases, would also have a significant role in deciding whether or not to save Naruto and how to go about it.