In the 'Fire Punch' light novel, the central character is Agni. He's the one with the special fire - punching power. Alongside him, there are antagonists and supporting characters. For example, Doma is a significant figure who has his own motives and actions that impact the story. And then there's Luna, who has a special connection with Agni and affects the plot in various ways.
The 'Fire Punch' light novel is about a guy who endures a lot in a frozen world. His power of the fire punch makes him unique. There are battles, both physical and mental. He has to fight against other tough characters and also deal with his own inner demons. It's a dark and gritty story.
Some Fire Emblem characters have the ability to use magic. If they were to combine their magic with Saitama's physical strength, they could potentially create new and powerful attacks. For example, a mage could cast a spell on Saitama to enhance his already incredible strength, allowing him to punch with even more force and cover a wider area. Also, characters with support abilities like healers could keep Saitama in top shape during long battles, which is something he probably wouldn't really need but it would be an interesting combination nonetheless.
Saitama is the main character. He is a bald - headed hero who can defeat any opponent with a single punch. He's a bit of an underdog in the sense that his power is so great that he's bored with fighting most of the time.
Another aspect of a crossover fanfiction could be the villains. The powerful villains from Fire Emblem, like Grima, might face off against Saitama. Grima with all its dark magic and power would still be no match for Saitama's one - punch ability. This would create a really interesting dynamic as the Fire Emblem characters would have to deal with the fact that their most feared enemy was so easily defeated by an outsider.
Genos is also quite popular. His loyalty to Saitama and his own journey to become stronger provide rich material for fanfiction writers. They can expand on his character development, his relationships with other heroes, and his attempts to understand Saitama's power.
Saitama is the most important character. He's the one - punch hero. Genos is also important. He's Saitama's disciple and has a cyborg body with powerful attacks. King is another key character. He's often mistaken for a very powerful hero when in fact he's just an ordinary guy who is lucky. Tatsumaki, the Tornado of Terror, is a very strong esper - type hero.
Genos is also a main character. He is a cyborg who becomes Saitama's disciple. He is very powerful but not nearly as strong as Saitama. He is constantly seeking ways to improve his strength and learn from Saitama.
Since I don't know the details of the 'One Punch of Justice Novel', I can't say for sure. But it's likely there is a main character with the one - punch ability.