In the story, the ballerina is an enigmatic figure. At Harrison's invitation, she stands up and volunteers to become his empress. 

The text does not explain why the ballerina decides to become Harrison's empress, however. However, we can refer to the content of the story to make deductions about why she does so.

From the story, we learn that Harrison is the one who removes the ballerina's physical handicaps and mask. The removal of the mask reveals a woman who is "blindingly beautiful." It is clear that the ballerina must have suffered immensely under the weight of her handicaps. The text tells us that, in Harrison's world, those who are physically and/or mentally gifted are forced to wear burdensome handicaps. 

It is likely that the ballerina chooses to be Harrison's empress in order to experience the freedom that has long been denied her. The text tells us that the ballerina takes a moment to make her decision. When she stands up, she sways like a willow, perhaps from a combination of nervousness and the weight of her handicaps. The ballerina's risky decision to become Harrison's empress is also a costly one. 

Like Harrison, she is shot dead by Diana Moon Glampers, the Handicapper General. However, the ballerina's courageous decision testifies to the human yearning for freedom and autonomy. The ballerina is willing to risk everything in order to experience a brief liberation from her burdens.