The Emperor's role is written for a mezzo-soprano voice – thus the abstract Emperor becomes the embodiment of evil. The use of typically romantic coloraturas for the role of Rachel is another device to remove a character from realism into abstraction – in this case, the sycophantic nature of Rachel.
The Emperor's role is written for a mezzo-soprano voice, making him an abstract representation of evil. Similarly, the use of typical romantic coloraturas for Rachel's part serves to distance her from reality and transform her sycophantic nature into an abstraction.