Jiraiya might be a supporting character. He was Naruto's mentor, and his knowledge combined with Itachi's could create an interesting dynamic. Also, Sakura could be involved. She might be curious about Naruto's training with Itachi and could offer her medical ninja skills in case of injuries during the training sessions.
Sasuke is often a supporting character. Since Itachi is Sasuke's brother, Sasuke's presence can add a layer of family drama and motivation for both Itachi and Naruto. Kakashi could also be a supporting character. He has a connection to both Naruto and has knowledge about Itachi, so he can play a role in facilitating the training or providing additional advice.
Shikamaru could be a supporting character as well. His intelligence could be used to strategize about Naruto's training progress. He might analyze Itachi's teaching methods and come up with ways to optimize Naruto's learning. And Hinata could be there, silently supporting Naruto during his training and maybe even learning a thing or two from Itachi's teachings.
Hinata is often a common supporting character. She might be the one who secretly helps Naruto when he is first banished, providing him with supplies or information. Another could be Kakashi, who may have doubts about Naruto's banishment and tries to find a way to bring him back. Shikamaru is also a possibility as he could use his intelligence to help Naruto navigate the difficult situation he's in.
Sasuke is often a common supporting character. He might have his own reasons for following or helping fem Naruto in exile. Maybe he feels a sense of loyalty or he too has issues with the village. Another could be Kakashi. He may secretly support fem Naruto and provide her with some guidance from afar.
Naruto himself is a very common supporting character. Since he has a connection with the Kyuubi, he often plays a role in the fanfiction. Sakura is also common. She might interact with the female Kyuubi, perhaps initially being afraid but then learning to understand her. Kakashi could be another one. His wisdom and experience can come in handy when dealing with the powerful and sometimes unpredictable female Kyuubi.
Sakura is often a common supporting character. She can be a great friend or even a rival to fem Naruto in these fanfictions. Sasuke is also very common, as his relationship with Naruto in the original series can be re - imagined in interesting ways with a female Naruto. Kakashi could also play a crucial role, perhaps as a mentor to fem Naruto on her journey to become Hokage.
Pikachu is a very popular supporting character. It's Ash's most iconic Pokemon and is always by his side during the training on Mt. Silver. Another could be Gary. Gary's rivalry with Ash can add an interesting dynamic, even if it's just in Ash's memories during his solitary training.
A wise old shaman from an Alaskan tribe might be a supporting character. The shaman could have deep knowledge of the spiritual side of Alaska and guide Percy in his training, perhaps helping him to connect with the forces of nature there.
Sakura could also be present. She might be observing the training and learning some things herself, or she could be helping Kakashi in some way, like providing medical support during intense training exercises.
One possible plotline could be Itachi teaching Naruto advanced jutsu techniques. For example, Itachi might show Naruto how to use his chakra more efficiently in combat, which could give Naruto an edge against stronger opponents. Another plotline could involve Itachi sharing some of the secrets of the Uchiha clan with Naruto, like the true nature of the Sharingan. This could lead to Naruto developing new strategies that combine his own abilities with what he's learned from Itachi.
Sakura could be a common supporting character. She might assist Naruto in his mission to save Rin. For example, she could use her medical ninjutsu to heal any injuries. Kakashi is also likely to be involved. He has a deep connection to Rin, so he would be a key figure in the events.
Sakura is often a common supporting character. She can use her medical skills to support Naruto and the village, and also offer him emotional support. Another is Kakashi. He has been Naruto's mentor for a long time and can offer his wisdom on being a leader.