Obviously Naruto himself. He is the main character and his growth and development during this arc are crucial. Then there's Zabuza and Haku. Their story of being outcasts and their complex relationship is very interesting to explore further. Tazuna also plays an important role as he is the reason Naruto and his team are in the Land of Waves in the first place.
Sasuke should be included as well. His rivalry with Naruto is a big part of the series. During the Land of Waves arc, we can see how Sasuke starts to respect Naruto more. Kakashi is also essential. His leadership and skills are on display, and his past experiences can be related to the current situation in the Land of Waves. For example, his own experiences with loss could be paralleled with the losses the people of the Land of Waves have faced.
Other important characters could be the enemies they faced during that era. For instance, the ninjas from other villages they fought against in the wars. These enemies could help showcase the Sannin's combat abilities and strategic thinking. Also, some of their comrades from Konoha, like Kakashi's father Sakumo Hatake, could be included to add more depth to the story.
Definitely Hiruzen. He was not only their teacher but also the Hokage at the time. His interactions with the Sannin, whether it was giving them advice or reprimanding them, were crucial. Then there's Dan. His death had a profound impact on Tsunade. In addition, characters like Shikaku Nara or Inoichi Yamanaka could be interesting to include. They were important members of the Leaf village and could have had some interactions with the Sannin, even if they were not as prominent at that time. It would give a more complete picture of the village's ninja population during that era.
Sasuke also plays a crucial role. His relationship with Naruto, whether it's one of friendship or rivalry in the end of the fanfiction, can shape the overall story. He has his own journey of redemption or further descent into darkness, depending on the plot.
Naruto and Hinata are obvious choices as they are central to the family. Their relationship and how they raise their children are always interesting to explore.
The Sage of Six Paths could be included. As he is the originator of many ninja powers, his connection to Amaterasu could be explored. Maybe he had a hand in creating or containing the power. His presence in the fanfiction would raise the stakes and give a more epic feel. Additionally, Hinata could be in the story. Her love for Naruto could play a role, for example, if Naruto is in danger because of Amaterasu, Hinata could try to save him in her own way.
Sasuke is a great character to include. His cool and brooding personality can create interesting dynamics in crossovers. For example, if he crosses over into the world of Attack on Titan, his skills as a ninja would be put to the test against the Titans. Another interesting character is Kakashi. His laid - back yet highly skilled nature can add humor and action to crossovers. He could be a great addition in a crossover with the world of Inuyasha, sharing his knowledge of combat and jutsu.
Kakashi is also well - liked. As Naruto's former teacher and a respected ninja in his own right, he can play a mentor role even when Naruto is Hokage. His laid - back yet highly skilled nature is fun to write about.
Gaara is a great character for 'naruto kage fanfiction'. He has an interesting backstory, going from being an outcast to a respected kage. His unique abilities and his friendship with Naruto can add a lot to the story. Tsunade is also popular. Her strength as a female kage and her medical ninjutsu knowledge can be interesting aspects to incorporate. And then there's A, the Raikage, with his powerful physique and no - nonsense attitude.
The Cyclops itself is of course the most important character. You could also include its parents if you want to explore its family origin. Another important character could be a wizard or a seer who has knowledge about the Cyclops' past and future.
A young girl who is an expert in water - based ninjutsu but was self - taught because she had no proper teacher in the Land of Waves. She could join Naruto's team temporarily and teach them some unique water - related techniques.