Well, it could be that Alfred is a main character. He's always been the one to take care of the Bat - family when they're unwell. So in this fanfiction about Robin being sick, he'd be there making sure Robin takes his medicine, cooks him nutritious meals, and provides words of wisdom.
His friends like Batman and Nightwing could be main characters. Batman might use his detective skills to figure out what's wrong with Robin, and Nightwing could be there to offer emotional support.
Since Rigby is sick, his family members could also be main characters in the fanfiction. They might come to visit him and bring their own unique family drama to the story. And his friends from the park, like Muscle Man and High Five Ghost, could also be involved, trying to cheer him up while he's ill.
Chica could also be a main character. She could be the one who tries to comfort Bonnie while he's sick. Maybe she brings him some virtual food or something to cheer him up. And Foxy might be another important character. He could be the one who goes on adventures to find a cure or a way to fix Bonnie. He's the more adventurous type, so it would make sense for him to be the one looking for solutions outside the normal areas in the pizzeria.
Well, the plot could involve Robin's friends rallying around to take care of him. Maybe they bring him soup and keep him company while he's in bed recovering.
Hanna could be one of the main characters. Since she and Caleb are in a relationship, she would probably be very involved in taking care of him and trying to find a way to make him better.
Obviously Elsa herself would be one of the main characters since she is the one getting sick. Anna would also be a main character as she would be very concerned about Elsa's condition and would do everything she can to help. Olaf could also play an important role, using his cheerfulness to try to make Elsa feel better.
Obviously, Bendy would be one of the main characters since he's the one getting sick. His friends like Boris and Alice would also be main characters as they would likely be involved in taking care of him or finding a cure.
Mario would most likely be a main character too. He's Luigi's brother, so he'd be there to take care of him or at least be really worried about him. Princess Peach could also be a main character. She might visit Luigi when he's sick and bring some kind of magical item that could potentially help him get better.
It could be Nightwing. He was once Robin himself and has a special connection to the current Robin. Nightwing might hear about the kidnapping and step in to save Robin, using his own unique fighting style and experience.
Well, Batman might become super protective. He could lock down the Batcave to keep Robin safe while he's sick.
Obviously, the main character would be the one who would become Robin. But there could also be other important characters like his trainers or guardians. For example, an old martial arts master who taught him the ways of combat.