Following the reports, SM Entertainment confirmed that members Seulgi and Irene were preparing to debut in the groups' first sub unit unit, with the debut date set for June 15, 2020. However it was postponed, on the grounds that it needed additional production on the album, in order to achieve a higher quality of music. On June 21, it was announced that they will make their debut on July 6 with their first mini album Monster.
Irene and Seulgi had their own reality show, Level Up Thrilling Project, which is a spin off show from Red Velvet's reality show, Level Up Project!. The show was filmed in Seoul and aired from July 8 to September 8.