He is a classically-trained baritone best known in musical theatre and has starred in Korean productions of A Tale of Two Cities, The Three Musketeers, Phantom, Jekyll & Hyde and Les Misérables.
[4][5] In a 2009 interview, Kai stated that he had shocked his professors and was met with opposition from most of them when he announced his intention to be a crossover singer rather than pursue a career in opera, as many of his peers had done.
[6] His decision was influenced by his professor, retired tenor Park In-soo, whose 1989 hit duet "Nostalgia" (향수) with folk singer Lee Dong-won is considered to be one of the earliest commercially-successful classical crossover Korean songs;[7] Park himself was expelled by the Korea National Opera and ostracized by the domestic classical music community after the song was released due to the staunch conservative and segregationist attitudes still prevalent at that time.
[8] He gained wider public recognition after featuring as a guest artist at the 2009 nation-wide concert tour of renowned Korean operatic soprano Sumi Jo.
At that time, there were relatively few classically-trained singers active in musical theater, with fellow Seoul National University voice majors Ryu Jeong-han and Kim So-hyun being the two most prolific and well-known.
Kai has been cast as authoritarian or "lofty" characters due to his powerful voice[18] and starred as the protagonist in the Korean productions of many notable musicals, including A Tale of Two Cities, Phantom, The Three Musketeers and The Count of Monte Cristo.