Well, not having read the 'naruto jinchuriki of the world eater fanfiction', it's all speculation. The Jinchuriki is bound to be a main character. If it's set in the Naruto universe, Kakashi could also be in it. He might be in a position of trying to protect the Jinchuriki from those who would misuse the 'world eater' power. There could be a group of rogue ninjas who are after the Jinchuriki's power, and they could be major antagonists. And there may be a mysterious character who has a connection to the origin of the 'world eater' and has their own agenda regarding the Jinchuriki.
The 'naruto jinchuriki of the world eater fanfiction' might center around the Jinchuriki's journey. Maybe they have to face challenges that come with being associated with a 'world eater'. This could involve battles against other powerful forces in the Naruto universe who either fear or want to use the power of the 'world eater' within the Jinchuriki. It could also delve into the Jinchuriki's relationships with other characters, like how Naruto's friends might react to this new and dangerous situation.
Obviously, Naruto Uzumaki is often a main character as he is the jinchuriki of the nine - tailed fox. Gaara, the one - tailed beast's jinchuriki, is also frequently featured. And sometimes, other jinchuriki like Killer Bee, who has the eight - tailed ox - like beast, can be main characters too.
Often, other Naruto characters like Naruto Uzumaki or Kakashi Hatake might be included in the two - tails jinchuriki fanfiction. Their interactions with the two - tails jinchuriki can create new and exciting plotlines, whether it's them helping the jinchuriki control their power or going on adventures together.
Yugito Nii is a very popular character as she is the actual 2 - Tails Jinchuriki in the Naruto series. In fanfiction, she often takes center stage, whether it's in stories about her training or her battles against powerful foes.
It could be Orochimaru. He's always been after power and Naruto having the power of two Bijuus would be very tempting for him. He might try to extract the Bijuus from Naruto to use for his own evil purposes.
It could be Orochimaru. He's always after power and seeing Naruto with Son Goku's power would make him a prime target. He might try to extract the power from Naruto for his own evil experiments.
Gaara is a very likely recipient. He was a Jinchuriki like Naruto and they have a strong bond. Naruto might send him letters to check up on him and continue their friendship.
A possible fanfiction could be centered around the Jinchuriki's role in a major battle. For example, in a war against an enemy ninja force, the Jinchuriki could unleash Matatabi's power in a strategic way to turn the tide of the battle. They might combine Matatabi's fire - based attacks with other ninja techniques they've learned, like water - style jutsu to create a steamy and powerful defense or offense. Also, their bond with Matatabi could give them an edge in terms of sensing the enemy's moves before they happen.
Well, Naruto is the central figure. Other important characters could include Kakashi. He might serve as an advisor to Naruto, using his wisdom and experience. And then there's Naruto's son, Boruto. He could play a role in the story, either as Naruto's heir - apparent or as a young ninja learning from his father's great achievements.
It could be the Akatsuki. Even though Itachi was once part of them, now that he has the power of a Jinchuriki, they might see him as a threat. They could want to capture him to either extract the tailed beast or to stop him from using its power against them.