Naruto himself is often involved. Since he has a strong will, it's interesting to see how he would deal with being possessed or using the possession jutsu. Sasuke is another common character. His dark past and powerful abilities make for great storylines when combined with possession jutsu.
Some fanfics might also feature Kakashi. He has knowledge of many jutsu and a past that is filled with secrets. So, he could be the one who either discovers a new aspect of a forbidden jutsu or is trying to prevent others from misusing it.
Sakura is another character that can be involved. Her strength and medical ninjutsu skills can be combined with other offensive or defensive jutsus. For instance, fusing her superhuman strength punch with a genjutsu to create a more complex and powerful attack. Her growth as a character throughout the series also gives room for different fusion jutsu ideas in fanfics.
Naruto himself is often a main character in such fanfiction as he is the protagonist of the original series and has a knack for learning new jutsus.
Naruto himself is often a main character in these fanfictions as he has a connection to the Hiraishin no Jutsu through Minato. Also, Minato is frequently featured as he was the original user of the jutsu. Sasuke can also be involved as he and Naruto have a complex relationship and their interactions can be explored in relation to the jutsu.
Naruto himself is likely to be involved. After all, he's always at the center of new jutsu developments in fanfiction.
Naruto himself is often a main character. Since the 'saimin no jutsu' is part of the Naruto universe, he could be either the caster or the victim of the jutsu. His unique personality and his own jutsu repertoire can interact in interesting ways with the sleeping jutsu.
Obviously Naruto is one of the main characters. And then there are usually some of the female characters from the Naruto series like Sakura, Hinata, and maybe even some of the female ninja from other villages. They are often part of the harem that revolves around Naruto.
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.