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.
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.
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'.
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.
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.
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.
Obviously, Loki would be one of the main characters. He could be joined by Thor, his brother. Thor might represent the 'light' in contrast to Loki's more complex nature. Maybe they have to work together to combat the encroaching darkness. Another character could be Odin, who might have some knowledge or power that is crucial in dealing with this situation.
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.