The important characters would start with Naruto. Then, any of the other Jinchuuriki that the author decides to include. For example, if the story is set in a certain time period, we might see the Jinchuuriki from Naruto's generation more prominently. So, characters like Gaara and Killer Bee would be very important. And there could be some new or less - known Jinchuuriki introduced to add more depth and variety to the story.
Obviously, Naruto is one of the main characters. And the other Jinchuuriki would also be main characters. For example, Gaara, who was the Jinchuuriki of Shukaku. There could also be Killer Bee, the Jinchuuriki of the Eight - Tails. These characters are important as they each have their own unique abilities and personalities.
There are many possibilities. Naruto could be in a situation where he has to rescue some of the other Jinchuuriki. Or perhaps they all gather in a secret place to train and improve their skills. It could also show how Naruto's positive attitude affects the other Jinchuuriki, leading to a change in their outlooks on life and their roles as Jinchuuriki.
Some fanfictions might also introduce new or less - known Jinchuuriki characters. They could be from different hidden villages or have special backstories. These characters add more variety to the story and can bring new dynamics to the relationships between Naruto and the other Jinchuuriki. Their inclusion can also expand on the lore of the Jinchuuriki in the Naruto universe.
Another character could be the Nine - Tailed Fox. Kushina was once the jinchuriki of the Nine - Tailed Fox. When Naruto meets his mother, the topic of the Nine - Tailed Fox might come up. Maybe Kushina could tell Naruto some secrets about controlling the Nine - Tailed Fox's power that she learned during her time as its jinchuriki.
Naruto is definitely one of the most popular. Since he is a central character in the original Naruto series and is also a jinchuuriki, he often takes center stage in fanfictions. His journey of befriending and mastering the power of the Nine - Tails makes him a very interesting character to write about.
Maybe Naruto would meet some young Uzumaki clan members who look up to him. They would be inspired by his strength and his will to protect the village. Naruto could then take on the role of a mentor, teaching them the values he learned in Konoha and also some of his own ninja techniques.
Perhaps there's a plotline where the other Jinchuuriki are in danger and Naruto has to rescue them. Since Naruto has a strong sense of justice, he would not hesitate to step in. This could lead to some intense battles and also show Naruto's growth as a character. He might use new jutsu or strategies he has learned along the way.
Obviously, Harry Potter himself is a crucial character. Then there could be an elder member of the other Potter clan, like an old wizard who has a vast knowledge of the family's history and secrets. There might also be a young Potter relative who is around Harry's age and becomes his new friend or ally.
One important character he could meet is the Raikage. The Raikage might initially be hostile towards Naruto due to his being from Konoha, but later sees his potential. Another could be Killer Bee. Maybe Killer Bee becomes Naruto's mentor in some ways, teaching him about the power of the tailed beasts in the Cloud village's perspective.
Since it's a Naruto - based fanfiction, Naruto is likely to be one of the main characters as he is the Jinchuuriki. There might also be other characters from the Naruto universe such as his friends like Sasuke, Sakura, etc. who could be involved in the story related to Naruto's connection with the Shinigami.