There could also be a mentor figure. This might be an old and wise wizard or an experienced slayer who has retired. They guide the main slayer, teaching them about the ways of the hellfire dragon and how to combat it. For example, they could show the slayer how to make a special potion that can protect against the dragon's hellfire for a short time. This character adds depth to the story as they have a wealth of knowledge from their past experiences.
Typically, the main character is the dragon slayer. They are often a brave and determined individual with unique skills and a strong will.
The hellfire dragon itself can be considered a main character in a sense. It's a powerful and fearsome being with its own motives and backstory. Maybe it was once a peaceful dragon until it was corrupted by dark magic, which turned it into the hellfire - breathing menace it is now. Understanding its origin can be an important part of the fanfiction story.