Lee debuted as a male model, becoming known for his feminine appearance and wearing women's clothing on the catwalk, but ultimately wished to pursue a career as an actor. He appeared in the films My Boss, My Hero (2001) and Sex Is Zero (2002), providing comic relief in effeminate roles, but felt pressured into cutting his hair and building up muscle. Lee recalled that, "once I entered showbiz, I was forced to live a life I didn't like", and growing increasingly unhappy, made several attempts at suicide before deciding to become a woman.
Adopting the given name Si-yeon, Lee underwent sex reassignment surgery in 2007, and admitted to having doubts over her sexuality since being in secondary school. She returned to acting later that year in the sequel Sex Is Zero 2, with her character from the original film now also a transsexual. Lee publicly announced her sex change prior to the release of the film, but was apprehensive about the reaction from audiences. Co-star Shin-ee observed that surgery hadn't changed her image much, and stated that she had always regarded Lee as a "kid sister".