There could also be some of Harry's wizarding friends like Hermione and Ron. Even though they may not be physically present at the Dursleys' all the time, they could play important roles. For example, they might be trying to find a way to get to Harry and free him. Their loyalty and friendship for Harry could be a major theme in the fanfiction as they strive to overcome the obstacles in their way to reach Harry.
Well, perhaps one plot could be Harry using his magic in a creative way while chained up at the Dursleys' to get himself free. Maybe he manages to communicate with his wizarding friends through some magical means and they come to his rescue.
The main characters would be Harry Potter without a doubt. Since he is the focal point of the story as he is chained up. The Dursleys are key characters too. Vernon Dursley, with his mean - spirited nature, Petunia, who has always been cold towards Harry, and Dudley, who might have a change of heart during the story. And of course, we can't forget about Hedwig. She could be a crucial character, perhaps trying to bring help to Harry or at least provide some comfort while he is in such a dire situation.
Voldemort or his Death Eaters are likely antagonists. They have always been out to get Harry, and chaining him up without his glasses would be a cruel and effective way to subdue him.
Obviously, Daisy Dursley herself is a main character. She is often the center of the story, with her actions and experiences driving the plot forward.
Maybe it's part of some dark magic plot against him. Some dark wizards might want to restrain him to prevent him from interfering in their evil plans.
One possible plotline could be Harry finding a way to break free from the chains and use his magic to confront the Dursleys about their mistreatment. Another could be the Dursleys realizing the error of their ways after chaining Harry up and having a change of heart. Or perhaps there are magical forces at work that lead to a series of adventures while Harry is still chained, like dealing with strange creatures that are attracted to the situation.
In many 'Harry Potter Dursley fanfiction' stories, the Dursleys are used as a contrast to the wizarding world. Their normal, mundane lives are juxtaposed with the magic and wonder of Harry's world. This can create interesting storylines where they are either forced to accept the magic or try to fight against it in their own stubborn ways.
Since it's a fanfiction, it could have the main RWBY characters like Ruby, Weiss, Blake, and Yang. They are likely to be central as the story is based on RWBY.
The main characters would be Harry and Lily. But it's possible that we could also see some appearances from the Marauders in flashbacks. Since Lily was friends with them, they could be part of the backstory. Also, Dumbledore might be involved in some way, perhaps as a guiding figure from afar, still looking out for Harry's well - being and the safety of the wizarding world.
Obviously, Vernon Dursley could be a main character. We could explore his thoughts and feelings more deeply, especially if the fanfiction takes a different turn on his personality, like making him more sympathetic.