Nina is an immensely talented ballerina, but equality unstable mentally. She seems to suffer from schizophrenia. Having dedicated her life to ballet, Nina's life consists of perfecting her ability and impressing everyone, yet is mind numbingly ascetic, especially with sex.
After meeting Lily at her dance academy, Nina's life was flipped. Lily represented every personality trait Nina lacked and Nina seemed mystified by her; possibly developing a crush.
Although Nina tried to distance herself from Lily, whom she viewed as a distraction and rival, the two eventually went out. After some drinks and taking ecstasy, Nina relaxed and had fun.
Returning home with Lily, Nina's lifelong repressed self-gratification finally exploded, and she seized Lily and began making out with her fanatically. The two girls stripped each other and the kissing eventually lead to sex, with Nina moaning deeply the whole time.