It's hard to say for sure without reading it, but it could be Naruto himself. Since it's a Naruto - related fanfiction, he might be the central figure dealing with the frozen flame concept.
There might be new characters introduced specifically for this fanfiction. Maybe a mysterious stranger who has a deep connection to the frozen flame. They could be the catalyst for the events in the story, guiding other characters in their exploration of this new power.
It's also possible that some of the existing side characters in Naruto, like Rock Lee or Shikamaru, take on more prominent roles in this fanfiction. They could be the ones who first encounter the frozen flame and start to figure out its secrets, leading to a change in their character arcs.
Since it's a Naruto fanfic, it's possible that Naruto Uzumaki is one of the main characters. He might be the one who first discovers the frozen flame or is drawn into the mystery surrounding it. Other likely candidates could be Kakashi, who could be guiding the younger characters in understanding this new and dangerous element. And perhaps there are some original characters created specifically for this fanfic that also play major roles.
Since I haven't read it, it's hard to say for sure. But it could be Naruto himself as he is the main character in the original series. There might also be an original character created for this fanfiction who has a connection to the dragon of the darkness flame.
One interesting concept could be the combination of Naruto's world with the element of frozen flame. Maybe it creates unique jutsus or powers. For example, a character might be able to use the frozen flame to both freeze and burn opponents simultaneously.
Some fanfictions might involve the Akatsuki. They could be after the Atlas Flame for their own evil purposes. Members like Itachi or Pain might be interested in using the power of the flame to further their plans of world domination or destruction, which would put them in direct conflict with the protagonists.
I'm not sure specifically as there could be many different takes in a Naruto fanfic with this title. It might involve some new adventure in the Naruto universe with a concept related to a frozen flame. Maybe it's a special power or an object that the characters have to deal with.
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.
Naruto and Hinata are common characters. Their relationship can be given a 'frozen' twist in fanfiction. Maybe Naruto is too focused on his goals at first and seems cold towards Hinata's feelings, creating a sort of frozen state in their relationship that needs to be thawed. Sasuke can also be a character in these. His brooding nature can be seen as cold, and his potential romantic relationships, whether with Sakura or someone else, can be explored in the context of 'frozen romance' as there are often barriers and coldness between them due to his past and his personality.
Sasuke is often a popular character in these fanfictions. His natural talent and brooding nature can be well - combined with the power of the flame bloodline, making for some interesting storylines where he has to control this new power.
Another possibility is Gaara before he met Naruto. He had a really tough childhood and was initially very cold - hearted, seeing others as tools or threats. His heart was like a frozen desert, until Naruto's friendship started to thaw it. In this fanfiction, they could explore more of his pre - Naruto state of a frozen heart and how it affected his relationships with others in the shinobi world.
There might also be side characters from the original that get more focus in the fanfiction. For instance, a servant who has a unique perspective on the events happening around the main characters and plays a more important role in this new fictional version.