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.
Well, it could be about the contrast between dark and light elements within the Yu - Gi - Oh universe. Maybe it focuses on characters who have a dark side but are also in a world filled with the ideals of light, like a hero who is tempted by darkness or a villain who has a glimmer of light in them.
I don't know for sure as I haven't read it. But it could be a protagonist who is struggling with some sort of inner turmoil or external conflict, and then there might be a secondary character who acts as the 'light'. For example, it could be a young wizard as the main character who is constantly battling his own dark magic tendencies, and then a wise old mentor could be the 'light' character.
I'm not sure as it depends on the specific fanfiction. It could be an original character created by the author who is trying to find their way out of a dark situation.
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.
Maybe it's an existing side character from the Yugioh series, like Bakura. Bakura already has a connection to the dark side in the original series. In this fanfiction, he could be further engulfed in darkness, becoming the 'aff'. His struggle between his good side and the dark forces that are taking over him could be the central focus of the story. He would have to use his knowledge of Yugioh dueling and his inner strength to break free from the darkness.
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 specifically as it depends on the author's creation. But it could be characters who are on a journey to understand the true nature of darkness and light, like a young adventurer or a wise old sage.
It could be about a character in Yu - Gi - Oh who is in a difficult or 'dark' situation and then something or someone brings hope or 'light' into their world. Maybe it's a story where a hero uses their cards to overcome great despair.
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.
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'.