There's no way to know precisely without reading the '3 days light novel'. It might have a complex set of characters including an antagonist who creates obstacles for the main character(s) during those three days. There could also be some supporting characters like family members or mentors who play important roles in the development of the story.
In the 'nijiiro days light novel', it's probable that the main characters are a diverse group. One might be a shy and introverted character who gradually opens up as the story progresses. Another could be an outgoing and energetic character who brings a lot of liveliness to the group. And there may be a character who is more mature and acts as a sort of mentor or guiding figure within the group. These characters' interactions and relationships are likely to be the core of the story.
I'm not entirely sure about all the main characters specifically, but there are usually characters like students who are at the center of the story's relationships and events.
Since we don't know the exact plot of the 'Seven Days Light Novel', the main characters could be anyone. They might be students if it's a school - based story. If it's a historical novel, they could be historical figures. There's also a possibility that the main characters are from another world if it's a fantasy story.
Often in a light novel like 'dog days', there could be a central character who is thrust into an unfamiliar world. Alongside this character, there may be native inhabitants of that world who become important main characters. These could be royalty, wise elders, or other individuals with their own motives and goals that interact with the protagonist in various ways.
One of the main characters is Shintaro Kisaragi. He plays a significant role in the story.
The main character is a man who sells his lifespan. There are also other characters he meets during his remaining days who play important roles in his self - discovery journey.
There's Kazuma, who is the central figure trying to navigate this strange new world with his rather odd group of companions. Aqua, the goddess, brings a certain level of chaos with her scatterbrained nature and water - based magic. Megumin, the explosion - loving mage, adds a lot of action and destruction (albeit in a limited capacity each day) to the story. And Darkness, the Crusader with her unique personality quirks, provides an interesting dynamic to the group. Their different personalities and abilities clash and combine to drive the story forward in the Konosuba Fantastic Days light novel.
The main character is Roxas. He is a key part of the story as it follows his time in Organization XIII. Another important character is Axel, who has a strong friendship with Roxas.
The main character is Kusunoki. He is a complex character who has reached a point in his life where he feels so desperate that he is willing to sacrifice his future years for money. As he embarks on this strange journey with his newly bought time, he meets other characters, perhaps people from his past or new acquaintances, but Kusunoki's story is the main focus. His transformation, his regrets, and his newfound understandings during those three days are what drive the narrative.
I'm not well - informed about this novel, but it's obvious that one of the main characters would be the stepsister. And the other one would be the person whose perspective the story is told from, probably a male character who is now her stepbrother. They are the central figures around which the story would revolve in 'days with my stepsister light novel'.