A mysterious stranger might be another main character. He seems to know something about the girl's past related to the dirty shirt. He could either be a threat or a helper in her journey, adding an element of suspense to the story.
There could be a kind - hearted old woman who takes the girl under her wing. She doesn't care about the dirty shirt and sees the potential in the girl.
Maybe a young boy who is also an outcast in some way. He and the girl in the dirty shirt form a bond and together they face the challenges in their lives, like bullying from other kids in the neighborhood.
Well, it could be about a girl facing hardships. Maybe she's poor and can't afford clean clothes, but she has a big dream. The story might revolve around how she overcomes the judgment of others because of her dirty shirt and strives for a better life.
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.
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.
Luigi could be an important character. He might notice the shy girl before Mario does and tries to encourage her to talk to Mario. Or, Toad could be there as a friendly sidekick, helping the shy girl get used to the new environment in the Mushroom Kingdom. He could also give her tips on how to overcome her shyness based on his own experiences.
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.
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.
His friends from his original life could be main characters. For example, if he has a best friend, that person would play an important role in helping Kuroko adjust to being a girl. Maybe they would offer support and advice, or just be there to listen when Kuroko has a tough time dealing with the changes.
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.
Edward Cullen could be one of the main characters. He could be the one who finds the girl when she wakes up and tries to protect her from the dangers of the Twilight world, like other vampires who might want to harm her.