I'm not sure about the exact plot as I haven't read it in detail. But generally, it might be about a hero who decides to stop fulfilling the role of a hero, perhaps due to some disillusionment or personal reasons.
Since I haven't read the 'I Will Quit Being the Hero Novel', I can only guess. There would probably be the hero himself who wants to quit. There might also be some friends or comrades who try to persuade him to stay in his heroic role, or who are affected by his decision. And there could be some antagonists who either oppose him because he's a hero or who take advantage of his decision to quit.
The plot typically involves a character who decides to leave the hero party for various reasons. Maybe they're tired of the constant battles, or they have different values from the rest of the group.
The main character is likely the one who decides to quit the hero party. There may also be other members of the hero party who play important roles, like the party leader who might be shocked or angry at the decision. And there could be new characters that the protagonist meets after leaving the party.
The plot often involves the main character's decision to leave the hero's party. This could be due to various reasons like feeling unappreciated, having different goals, or being tired of the hero's journey. Usually, after leaving, the character embarks on a new path that might cross with the hero's party again in unexpected ways.
Perhaps the writing style changed. If at first it was engaging and then became too convoluted or simplistic, that would be a reason to quit. Also, if new elements were introduced that didn't fit with the original concept of the light novel, it could turn you off.
In 'I Quit the Hero's Party' novel, the protagonist who decides to leave the party is a key character. There might be a leader in the hero's party who has a particular relationship with the quitting character. Maybe there are also some side characters who are friends or foes of the main character. These characters would contribute to the story's development, whether it's through their reactions to the quitting or their interactions with the main character as they pursue their new path.
One reason could be disillusionment. Maybe the character witnessed the dark side of heroism, like politics involved in it. For example, they saw that their heroic deeds were being used by corrupt leaders for their own gain.
It could be that you faced an overwhelming enemy or a situation that made it impossible to continue as the Demon Lord. Or maybe you just wanted a simpler, more peaceful life.