On November 6, 2005, Leeteuk officially debuted on SBS's Popular Songs music program as a member of Super Junior 05, the first generation of Super Junior. The group performed their first single "TWINS (Knock Out)" to an audience of about 500 fans.
Leeteuk explained that he uses a stage name due to his desire to have a name with the same effects as Kangta (and to also prevent confusion with veteran actress Park Jeong-su). His stage name denotes being "special" as he wants to be a special member in the group.
Leeteuk also became known as the "special leader" (특별한리더) of the group. Not only becoming a "special leader", but also having a sweet voice as a tenor of Super Junior. He is also noted for having a unique and goofy laugh. Sumber : Wikipedia