Kakashi might also be there to assist Naruto. His wisdom and combat skills are always useful. He could provide strategic advice and even jump into battle when needed.
Shikamaru might be involved. He's very intelligent and could come up with a clever plan to outmaneuver the Akatsuki. His shadow - binding jutsu could also be used to immobilize some of the Akatsuki members.
The demihumans themselves would be important characters. There could be a demihuman leader who Naruto teams up with.
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, 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.
Sakura could be a common supporting character. She might assist Naruto in his mission to save Rin. For example, she could use her medical ninjutsu to heal any injuries. Kakashi is also likely to be involved. He has a deep connection to Rin, so he would be a key figure in the events.
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.
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.
A common villain could be Orochimaru. Since he was always interested in powerful jutsus and abilities, he might target Sakura to use her as a pawn in his experiments. And Naruto, with his demon powers, would have to stop him. Orochimaru is a well - known and menacing character, so it makes sense for him to be a villain in these fanfictions.
Pain is often one of the main antagonists. Since in the original story, Pain was the one who ultimately killed Jiraiya. So in fanfictions where Naruto saves Jiraiya, Pain is a natural choice to be the bad guy. Another common antagonist could be Orochimaru. He had his own vendetta against Jiraiya and the Hidden Leaf Village. He might be trying to use Jiraiya for some evil experiment or to gain access to the secrets of the Sage Mode.
In a 'Naruto Needs Help' fanfiction, Naruto could get help from unexpected sources. The tailed beasts themselves could come to his aid. Since Naruto has a connection with Kurama, the other tailed beasts might sense his distress and offer their power. Also, characters from other villages. For example, Killer Bee could teach Naruto some new techniques related to the tailed beasts' power, or Temari could use her wind techniques to assist in a battle where Naruto is outnumbered.