An alchemist who is trying to study and harness the power of the dragon's fire could be a main character. They might be on a quest to find a way to use the fire for good, like curing diseases or creating powerful magic items.
There could also be a dragon rider. This character forms a deep bond with the fire dragon and they fight side by side. The rider might have a special skill or power that complements the dragon's abilities.
I don't know for sure, but I assume the fire dragon with tattered wings is one of the main characters. There could also be other dragons, maybe a healer dragon who tries to fix its wings.
A dragon tamer could be the main character. They have a special bond with the dragon and use the dragon's roar to their advantage in battles or to solve problems.
It could be a rogue ninja who has also mastered the Fire Dragon Roar but uses it for evil. This ninja might have a grudge against the Hidden Leaf Village and wants to destroy it using his powerful fire - based attacks. Maybe he was once rejected from the village and now seeks revenge.
It could be a dark ninja who wants to control the power of the fire dragon for his own evil purposes. He might try to steal the dragon's chakra or use it to destroy villages.
A powerful dark mage who wants to use Lucy's newfound dragon slayer powers for his own evil purposes. He might try to control her or steal her powers.
Perhaps an evil mage who has been researching on how to control dragons. When he hears about Natsu's transformation, he sees Natsu as a perfect specimen to experiment on. So he becomes the main antagonist, trying to capture Natsu and use his powers for his own dark purposes.
It could be an evil dragon slayer who is jealous of Natsu's new form. This antagonist might try to steal Natsu's power or find a way to turn him back to his normal self out of envy.
Well, typically the main character would be a fire dragon slayer. They are the ones with the special connection to the fire dragon powers.
The main characters could be a young, inexperienced trainer who has just caught their first Fire - type and Ice - type Pokemon. As they journey through the Pokemon regions, they learn about the strengths and weaknesses of their Pokemon, and grow as a trainer. Along the way, they meet other trainers and Pokemon that help or hinder their progress. For example, an older, more experienced trainer who tries to convince the young trainer to give up their Ice - type Pokemon because it's 'too weak' in their opinion.