The main character would likely be the girl herself who has been behaving badly. Maybe she's a young girl who is going through a rebellious phase.
Friends could also be main characters. They could be the ones who are affected by her bad behavior, or they could be the ones trying to help her see the error of her ways. For instance, if she has been stealing from her friends, they might confront her about it and try to make her change.
There could also be other supporting characters. For example, if it's set in a school - like fanfiction setting, there might be classmates or teachers who are affected by the 'bad girl's actions or are involved in the consequences she faces. Or if it's in a more supernatural setting, there could be other magical beings who play a role in how the 'daddy' deals with the 'bad girl'.
In such a story, the consequences could be things like facing disappointment from loved ones. For example, if a girl has been lying a lot, her parents might be sad and lose trust in her.
Well, there must be the bad girl herself. She's likely at the center of the story, causing all kinds of trouble or having a negative impact on those around her.
Fanfiction often takes existing characters or concepts and spins new tales. In 'daddy i've been a bad girl fanfiction', it might be that the 'bad girl' behavior is related to disobeying an authority figure represented by 'daddy'. It could be in a fantasy setting, like a magical realm where she didn't follow his instructions regarding magic use or something similar.
There could be a lead 'bad girl' who is the most rebellious and fearless. She might be joined by her friends or other girls who she meets on the island. Their personalities could range from sassy and outspoken to cunning and resourceful, and together they form the main characters in this 'bad girl island story'.
Obviously, the main characters are the good boy and the bad girl.
There could also be a male character who is caught in the middle of the good girl and the bad girl. He might be attracted to both of them for different reasons. He could be a jock who is used to getting attention from girls, but the good girl and the bad girl present him with different challenges. He has to decide which one he really wants to be with, and this decision could be based on his own growth and values.
I'm not sure exactly as I haven't read it, but surely the bad girl is one of the main characters. There might be a best friend character who either supports or clashes with her.
In the 'Gossip Girl' series, Serena van der Woodsen is one of the main characters. She is often seen as a 'bad girl' as she has a wild past and is involved in various romantic and social dramas. Blair Waldorf is another key character. She can be quite mean - spirited at times in her pursuit of status and love, often engaging in schemes against her friends and rivals.
Since I haven't read this particular Wattpad story, it's hard to say exactly. But likely, the main character is the girl who goes from good to bad. There might also be a best friend who tries to stop her from changing or a love interest who either encourages or discourages her new behavior.