Obviously, the good girl would be one of the main characters. She's the one whose transformation we are following. And the dog, of course, is also a main character as it seems to be the catalyst for her change. There could also be other characters like her friends or family who react to her new behavior and try to bring her back to her old self.
The plot might involve the girl having a normal life at first. Then she meets this dog which has some extraordinary abilities or is part of some strange event. As she documents her experiences with the dog on her blog, her actions start to deviate from what people expect of her. She might break some rules or social norms because of her relationship with the dog, thus going 'bad'.
One idea could be that the dog in 'Dog with a Blog' witnesses a character from the show starting to act out in a 'Good Girl Gone Bad' way. Maybe the sister character starts to rebel more, and the dog blogs about his confusion and attempts to get her back on track.
You could have the girl get into trouble with the law in a small way, like getting caught spray - painting graffiti. The dog would be shocked and write about it on his blog. He might also try to understand why she did it. Maybe she felt ignored at home or wanted to be part of a 'cool' crowd. The family's reaction to the dog's blog entries could be an important part of the story as well.
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.
Probably the main character is the girl - turned - queen. There could be her family members at the start who represent her good life, like her parents or siblings. In the underworld, there may be powerful criminals or leaders who she has to interact with. Maybe there's a love interest as well, someone who either supports her new life or tries to bring her back to her old self. And of course, there would be her minions or followers once she becomes the queen.
Since I haven't read the novel, I can't say for sure. But based on the title, the main character is likely to be the girl who goes from good to bad and becomes the queen of the underworld.
The dog himself is an obvious choice. You can delve deeper into his thoughts and feelings as he navigates his double life of being a pet and a blogger.
Most likely, the main character would be the dog. But there could also be the dog's owner or other people that the dog interacted with.
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 don't know who the author of 'bad girl gone a novel' is as the title alone doesn't give that information.