Seo was born on October 23, 1987, in Ulsan. He grew up in relative poverty, his mother worked as a recyclables collector and his Father a welder. He has a younger sister. Seo trained as a ssireum Wrestler, learnt boxing and mixed martial arts in school and is a Hapkido 2nd dan. He decided to become a singer at the age of ten, after being inspired by the Korean rock musician Kim Jung-min. He performed at family gatherings and school events, and went on to study Applied Music at Daebul University, also auditioning in entertainment agencies. After repeatedly being rejected and told to lose weight, he struggled with bulimia for a time.