Sejin

On July 22, 2017, Sejin made his debut in the boy group 14U with the single album "VVV". He was a member of the group until their disbandment on May 13, 2019. On November 20, 2019, Sejin was revealed as a member of the pre-debut group UTH.

However, according to Instagram posts made by him and Kosuke, the group disbanded in early 2020 before even having an official debut. On November 6, 2020, C On H Entertainment announced they were suspending all activities of the group and would be re-organising internally. He unfollowed the company's Instagram account and, alongside two other members of the group, joined Floret Entertainment.