Choi Seok-won (최석원), better known as Tan, is a South Korean singer under Rain Company. As a trainee, he appeared in the 2015 Starship survival program "No.Mercy" and competed to be a member of Monsta X. After being eliminated in the show, Tan went on to enlist in the military. After being discharged, he continued his journey as a trainee.
In 2021, Tan made his official debut as the main vocalist of the boy group CIIPHER.