Perhaps there's a mysterious figure in the background who actually caused the transformation on purpose for some unknown reason. And then there are bpy's classmates. Some of them might be curious, some might be mean, but there could be one or two who become really good friends with bpy as a girl and they all go through different adventures together at school or in the neighborhood.
There could be a wise old wizard who tries to reverse the transformation or at least guide bpy through the process of being a girl. Another character might be a best friend who accepts bpy immediately regardless of the change and helps bpy with all the girl - related stuff like choosing outfits.
Well, perhaps bpy is in an experiment gone wrong in a science - fiction setting. The experiment causes a biological change that turns bpy into a girl. At first, bpy is confused and scared. But as time goes on, bpy starts to explore this new identity. She might make new friends who help her understand what it means to be a girl. She could also get involved in activities that are more common for girls, like dancing or art, and in the process, find a new sense of self.
A love interest who breaks the girl's heart can also be a main character. His actions, like cheating on her or suddenly leaving her without an explanation, can be the catalyst for her transformation into an evil character. Then the story could revolve around her trying to get back at him in devious ways.
Another possible main character could be Frisk. Since Frisk has a connection with Sans, they would be involved in Sans' story when she becomes a girl. Frisk could be the one who first discovers Sans' transformation and helps her come to terms with it, or they could be on an adventure together with Sans in her new form.
Pikachu would definitely still be there. It's Ash's most loyal companion, and it would be interesting to see how their relationship might change now that Ash is a girl. Maybe Pikachu would be more protective or show a different kind of affection.
Besides Steven, the main characters could include the villains he's faced in the past. Now that he's a girl, they might underestimate him or treat him differently. His father Greg could also play a big role. Greg might be a bit shocked at first but then would be super supportive, trying to understand what his 'new' child is going through. And there could be new characters introduced, like other girls in the town who become friends with Steven and help him adapt to the female world.
Lucy could also play a major role. She might be the one to help Natsu adjust to his new life as a girl. Maybe she gives Natsu tips on how to dress and act like a girl, and they could have deeper conversations about what it means to be a girl in their magical world.
Maybe the green cloak represents a certain identity or group in the school in the novel. It could be a symbol of a special club or a status among students.
Maybe there's a mysterious visitor who comes to the ward from time to time. This person seems to know something about the girl's past or has some connection to her that she doesn't remember at first. As the story unfolds, this visitor becomes a key figure in the girl's life in the ward.
There could be a friendly teacher. This teacher notices the new girl's struggle to adjust and offers her support and advice. For example, the teacher might give her tips on how to make friends or deal with schoolwork.
Latios would obviously be a key character. Also, Pikachu, Ash's loyal Pokémon partner, would be there. Maybe Misty or Brock could also be part of the story, with them either supporting or being skeptical of Ash's newfound admiration for Latios.