Sasuke is often involved. Since he and Naruto have a complex relationship, his reaction to Naruto's demon form can be a major plot point. He might be initially suspicious or even see it as a new way to compete with Naruto. Sakura could also be there. She might be worried about Naruto and try to find a way to help him control this new form.
Another character that could be involved is Gaara. Since Gaara has his own experience with being a host of a demon - like entity (Shukaku), he might be able to relate to Naruto's situation. He could offer advice or even form an alliance with Naruto based on their shared experiences. And of course, villains like Orochimaru might be interested in Naruto's demon form and try to either steal it or use it for their evil plans.
Sasuke is likely to be involved. Given their complex relationship, Sasuke might be on the receiving end of Naruto's rage at times or be someone who tries to understand and calm him. Kakashi could also be there, as Naruto's sensei, he would surely be involved in dealing with Naruto's rage.
Obviously Naruto is often a main character in 'Naruto Crystal Fanfic' as it is related to the Naruto universe. Sasuke can also be involved, perhaps having a different view on the power of the crystal compared to Naruto.
Sasuke is often a main character in 'eye naruto fanfic'. Since he has the Sharingan, many fanfics explore different aspects of his eye power, like how he might develop it further or use it in new and unexpected ways. Another one is Hinata. With her Byakugan, fanfic writers can create stories about her using it in battles or for some other special purposes, like finding hidden secrets in the ninja world.
Naruto himself is often involved. Imagine a fanfic where Naruto finds a strange mask that gives him new powers, but also has some negative side - effects. Sasuke can also be a main character in such fanfics. He might be on a quest to find a mask that has a connection to his clan's history.
The Kazekage is the central figure in 'naruto kazekage fanfic'. But we also see characters like Sakura. She is a strong kunoichi and her medical skills could come in handy in a story set in the Naruto world. Another character often involved is Shikamaru. His intelligence could play a crucial role in any plot related to the Kazekage. And of course, the Sand Village elders might be important characters as they have influence over the Kazekage and the village's decisions.
Kakashi Hatake is often a main character in 'raikiri naruto fanfic' since he is the one who originally developed the Raikiri. His skills and the way he uses the Raikiri can be further explored in fanfics.
Naruto himself is often involved. He might use bubblegum in his usual impulsive and creative ways, like coming up with new pranks or battle strategies. Sakura could also be a main character. She could use her intelligence to develop new types of bubblegum or figure out how to counter an enemy using bubblegum. Sasuke might be involved too, perhaps reluctantly at first, but then finding that bubblegum has some unexpected uses in his quest for power.
Kakashi can also be involved in some shukaku naruto fanfic. He could be Naruto's mentor in understanding the Shukaku better. He has a wealth of knowledge about the ninja world and could offer advice to Naruto on how to handle the power of the Shukaku and any potential risks associated with it.
Naruto is often involved. Since he's the main protagonist, writers like to put him in situations where he has to deal with deception, either being deceived or trying to deceive the enemy. Sasuke is also a common character in such fanfics. His complex personality and his past actions make him a great target for deception plots or a character who could be doing the deceiving.
Often, Naruto himself is involved. Since it's in the Naruto universe, he might be the one to encounter the Demon of the Mist and try to figure out what to do about it. Also, characters from the Mist Village like Zabuza or Kisame could be main characters as they have connections to the Mist and its secrets.