It could be the descendants of the original Naruto characters. For example, Naruto's son might be one of the main characters, carrying on his father's legacy and the will of fire.
There might be new characters created specifically for the fanfiction. These could be students of the old - time heroes. They are inspired by the tales of Naruto and his friends and strive to uphold the will of fire. They may have their own unique abilities and personalities, but are united in their belief in the will of fire. For instance, a young girl with a special healing jutsu who uses it not just to save lives but also to inspire hope, which is an important part of the will of fire.
Maybe some characters from other villages who have been influenced by the concept of the will of fire. They come to Konoha to learn more about it and end up becoming part of the story. They add a new dimension to the fanfiction as they bring in their own village's values and compare them with the will of fire, creating interesting interactions and conflicts.
Typically, the main characters are the inheritors themselves. They could be young individuals who have just discovered their inheritance and are trying to come to terms with their newfound powers and responsibilities.
One interesting story could be about a new generation of ninjas who are trained in a unique way to inherit the will of fire. They might face challenges that test their resolve and loyalty to the village.
Lok is a central character among the inheritors. He is rather innocent and tries to understand the new and strange things he encounters. Fa is his female counterpart. She has her own way of reacting to the changes. And the new humans, who are more advanced in technology and culture, are also main characters in a sense as they drive the plot through their actions and interactions with the inheritors.
Naruto is surely the central character in this fanfiction. Besides, characters like Kakashi could be involved. He might be Naruto's mentor in controlling and understanding his new 'king of fire' powers. And villains from the Naruto universe might also appear. They could be envious of Naruto's power and try to take it away or use it for their own evil purposes. So, a variety of characters from Naruto's world could be main characters in this fanfiction depending on the plot.
Since there are many 'guardian of fire' Naruto fanfictions, the main characters can vary greatly. It could still have Naruto as a main character, along with the new 'guardian of fire' character, who could be an original creation for the fanfiction.
It's possible that the main characters could also include some of the antagonists from the original series. Maybe they are behind the cause of the fire and ash, or they take advantage of the chaos to further their own goals. Naruto and his allies would then have to face these enemies while also dealing with the desolate environment.
I'm not sure specifically as I haven't read this exact 'the inheritors novel'. But generally, there could be an heir - a character who is set to inherit a large sum of money or a grand estate. This character might be young and naive at first, and then grow as they face the challenges of inheritance.
Obviously, Naruto and the Fire Daimyo are the main characters. Naruto is the well - known ninja from Konoha, and the Fire Daimyo is the leader of the Fire Country.
Naruto himself could be a main character. He has a natural inclination towards befriending powerful beings, and his determination would make him a great fit to interact with the fire and ice dragons. Sasuke could also be a main character, with his Sharingan potentially having some unique interactions with the dragons' powers.
There could be a whole new set of characters in the 'naruto frozen fire fanfic'. It might be a group of rogue ninjas who have discovered the secret of frozen fire and are using it to their advantage. Or it could be a young apprentice who is learning to control this strange power under the guidance of an old sage. Without reading the actual fanfic, it's all speculation though.