@reu
It's outright stated that gender and sex can be different and that they likely can't be changed by magical means. Applying trans logic to this story makes sense because the fact we exist is brought up by the text itself in a way that clearly references Rei. On top of that, we know that Rei is incredibly dense and does enjoy being a girl when she is allowed to forget she was ever a guy she is more than happy to be as girly as she wants to be. See the part where Arsenio makes a fake spell so she can enjoy girl time with Betty. Something she was so eager to try. She wanted to be a normal girl. There are similar examples throughout the story of her being sad about not being a "real girl" for example when it came to her engagement to Lloyd. Every justification for why she wants to be a guy is rooted in outside forces, rather than anything intrinsic to her, she wants to be a guy to take over the family business, she wants to be a guy for her family, she wants to be a guy so she can live up to her families expectations of a dragon hunter. Even if that's not what she wants.
There is a very good case for her being a trans girl who never realised she wanted to be a girl before she was turned into one.