Well, obviously Joseph and Brooklyn are main characters. But often they are associated with other well - known MLP characters like Twilight Sparkle. Twilight might play a role in guiding them or being part of their adventures. Pinkie Pie could also be involved, bringing her element of fun and spontaneity to the story.
One common theme is the exploration of different relationships. Joseph and Brooklyn might be depicted in various kinds of relationships within the fanfic. It could be a mentor - mentee relationship, where one character teaches the other valuable lessons. Another theme could be the struggle against evil. They could be teaming up to fight against a dark force that is threatening their world, which often involves themes of courage and loyalty.
Well, obviously Discord is one of the main characters. He brings his chaotic and powerful nature to the story. Then there's the 'bride' character, who could be an original creation or an existing character from the MLP universe given a new role. For example, it could be a mare who catches Discord's eye in a unique way.
Princess Celestia is also commonly included. Since it's a Canterlot wedding, as the ruler of Canterlot, she would play an important role in the organization and significance of the event. She could be shown making important decisions regarding the wedding.
Well, obviously Clarisse Joseph is a main character. Depending on the plot, there could be her best friend, let's say a character named Lily. Lily could be the one who supports Clarisse through all her adventures. Another main character might be a love interest, perhaps a guy named Ethan. He could be the one who challenges Clarisse and also shares in her joys and sorrows.
His family members could be main characters. For example, his parents, David and Victoria Beckham. They would be worried about him and try to do everything they can to help him get better. Maybe they would call in the best doctors or stay by his bedside all day.
Obviously Bonnie is a main character. And often, there will be a Pokemon that she has an obsession with. It could be a popular one like Pikachu or Eevee. There might also be a trainer who becomes the target of her yandere behavior.
Eilis is one of the main characters. She is the protagonist whose life in Brooklyn is the central focus of the story. She experiences many changes and challenges as she moves to a new country.
I'm not entirely sure without rereading it, but I think there's a young Irish woman who is one of the main characters.
Often, there are characters who are sweet - lovers. Maybe a young girl or boy who has a special affinity for chocolate and is at the center of the story.
Sometimes, the main characters could be metro workers. For instance, a young metro driver who discovers something unusual during his shift. Or it could be a maintenance worker who stumbles upon a hidden part of the metro system while doing repairs. Another type of main character could be a street artist who uses the metro stations as their canvas and has a whole story related to their art and the metro environment.