Another character could be Kakashi. He might be jealous or curious about Naruto's new apprenticeship with Hiruzen. He could pop in from time to time to check on Naruto's progress or even offer some advice based on his own experiences as a ninja.
Obviously, Naruto and Hiruzen would be the main characters. Naruto as the apprentice and Hiruzen as the mentor.
There could also be other characters like Iruka. Since Iruka has been an important figure in Naruto's life, he might be involved in some way. Maybe he helps Hiruzen understand Naruto better at the beginning of this apprenticeship.
The main characters would be Naruto and Hiruzen. However, we might also see characters like Kakashi make an appearance. Kakashi could be a bit jealous at first that Hiruzen is taking Naruto under his wing, but then he might see the benefits and offer his own advice to Naruto on the side.
Obviously, Naruto and Hayate would be the main characters. Naruto is the protagonist who will be learning from Hayate.
Obviously, Naruto and Anko would be the main characters. Naruto, the energetic and determined ninja, and Anko, the somewhat eccentric and skilled kunoichi.
Anko might also be involved in the story. She has her own unique relationship with Yugao and could potentially interact with Naruto. Maybe she gives Naruto some tough love or challenges him in a way that Yugao's teaching methods don't cover. And of course, Iruka sensei could also show up to check on Naruto's progress and have some interactions with both Naruto and Yugao.
In 'harry is an apprentice fanfiction', apart from Harry, there might be other apprentices too. They could be his friends or rivals during his apprenticeship. Also, we can't rule out the appearance of some of the main antagonists from the original series, as they might try to interfere with Harry's apprenticeship journey.
The plot might involve Naruto learning about the history and politics of the village from Hiruzen. Since Hiruzen has a wealth of knowledge and experience, he could be guiding Naruto on how to be a great leader in the future, not just a powerful ninja.
Another possible main character could be a new villain. Since Harry is in a new role as an apprentice, there could be someone who wants to steal the secrets of Ollivander's wand - making. This villain could be a dark wizard or a rogue wizard who has his own motives for wanting to control the wand - making process. Harry would then have to protect Ollivander and the shop from this threat.
There could also be other characters from the Naruto universe that play important roles. For instance, Tsunade might be involved as she has a connection to Jiraiya. She could offer additional guidance or be a source of conflict when she disagrees with Jiraiya's training methods for Naruto.
Perhaps there are some shinobi within the village who are jealous of Naruto's potential. They might try to harm Naruto in secret, and Hiruzen has to be vigilant against them. These shinobi could be from clans that have rivalries with the Uzumaki or those who just don't want a new powerful force emerging in the village.
Obviously Naruto would be one of the main characters. He's the protagonist of the original series and his power and will would be crucial in a doomsday scenario. Sasuke would also be likely, as his strength and his connection to Naruto are important elements. Sakura, with her medical skills, would also be a key character as she could be responsible for healing the injured during the doomsday chaos.