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.