The group consists of seven members: Sumling Li, Garie Shum (Gao), Winka Chan, Ivy So, Rejena Simkhada (Day), Marife Yau (Marf), and Candy Wong.

[2] In July 2018, ViuTV's reality talent competition series King Maker [zh] started scouting for potential artists, and after three seasons, three boy bands, namely Mirror, Error and P1X3L were formed, and some contestants debuted as solo singers.

On 12 January 2022, 8 contestants from King Maker IV were announced from the Top 20 of the season, formed the group through a press conference named "Hold Your Breath" which held in Harbour City.

In the press conference, the name of the group "Collar", along with five other members, Sumling Li, Natalie So, Winka Chan, Ivy So and Candy Wong, were announced in the lineup.

[15] On 10 May 2023, Natalie So announced her leaving from the group in a statement of social media, stating that her mental injury suffered from her boyfriend Moses Li after the 2022 Mirror concert incident [zh] had caused trauma that “could not be healed”, and she was unable to cope with the show business.