Yeon Kyoo Seong (연규성, born March 1, 1980, in Cheongju, Chungcheongbuk-do) is a South Korean singer, songwriter, and vocal trainer celebrated for his powerful voice and enduring influence in the underground rock and R&B scenes. Since debuting in underground live clubs in 1999, he rose to prominence as a legendary presence in the online community “Rock Town 21,” inspiring many future vocalists. His standout performances, including “Sacrifice” at the 2003 Yonsei Romantic Eagle Festival, and his perseverance through spasmodic dysphonia to reach the Top 10 of Superstar K4 in 2012—where his duet rendition of "Malli Flower" with Hong Dae-kwang became iconic—have cemented his reputation. Today, Yeon continues to nurture singers as a respected vocal coach and shares his artistry with fans through YouTube covers. In July 2025, Yeon expanded his musical repertoire by lending his voice to the OST of the mystery‑thriller film The Grace of the Teacher (영화 스승의 은혜) with the release of “Stop killing me down” on Movie ‘The Grace of the Teacher’ Take. 2 by DSP Media.