I'm not sure specifically as we don't have more details about the story. But it could be an ordinary person who becomes a hero, like a young girl who finds a cure for a deadly disease in a world full of suffering. She would be one of the main characters.
Perhaps there are two main characters. One could be a lost traveler in a desolate land, and the other could be a wise old hermit who shows the traveler the way out, both of them playing crucial roles in the 'light in the darkness' narrative.
There might be a group of main characters. For example, in a story about a city facing a mysterious darkness that is making people ill. A doctor, a scientist, and a brave journalist could be the main characters. The doctor tries to find a medical solution, the scientist investigates the cause, and the journalist spreads awareness. Together, they are the lights in the city's darkness.
I don't have enough information to say who the main characters are as I don't know the particular 'a light in the darkness story'.
Since it's a Twilight fanfiction, it might focus on the Cullens as the main characters. One of them could be the one to bring the 'light in the darkness'. For instance, Carlisle with his moral compass and leadership could be the central figure in a story about guiding the family through a difficult time in the Twilight - like world. Or perhaps it could be a new Cullen - like vampire introduced in the fanfiction who plays this role.
It's hard to say exactly who the main characters are without reading the fanfiction. But if we consider the themes of 'darkness in the light', it could be characters who are on the verge of losing their way in the light - filled Yu - Gi - Oh world. It might be some of the minor characters from the original series who are given more depth in this fanfiction. For example, a character who was always in the background but now is in the center of a story arc dealing with the discovery of a dark power within a seemingly innocent card. Or it could be an entirely new character created by the fanfiction author who has a unique connection to both the light and the darkness in the Yu - Gi - Oh universe, and through their journey, we explore the concept of 'darkness in the light' more deeply.
It's hard to say without reading it. But it could be the usual Twilight characters like Bella, Edward, and Jacob. Maybe it also features some minor characters from the original series in more prominent roles.
Kazuma is one of the main characters related to Darkness. Their relationship is full of comical exchanges. Also, Aqua and Megumin are important characters in relation to Darkness. They are all part of the same party in the story.
Maybe there is a protagonist who has some special ability to combat the darkness, and there could also be some side characters who either help or hinder the protagonist's journey in the 'the darkness 1 story'. Without more context, it's difficult to be more precise.
Since I don't know the details of the 'darkness 2 story', I can't say who the main characters are.
Since I don't know the exact 'the darkness story', it's hard to say. But it could be a lone hero who has to face the terrors of the darkness. Maybe it's a young child who is the only one seeing the strange happenings in the dark.
In this fanfiction, aside from the core RWBY team, there could also be other important characters. Ozpin might play a role as he often has knowledge and guidance to offer. Qrow Branwen could also be involved as he is Ruby's uncle and has his own set of skills and connections. Additionally, characters like Pyrrha Nikos might make an appearance if the story is set in a timeline where she still exists, and she could be a powerful ally in the battle against the darkness.
I'm not sure as I haven't read it. But usually in such novels, there could be a brave hero or heroine who leads the fight against the darkness.