A member of a popular rookie girl group became the first one to appear on 'Mask King'!

On the February 24 episode of the show, 'Enter Lady' went up against 'Play Guy' for a chance to compete against the reigning 'Mask King,' mystery contestant 'Klimt'. 

The two contestants performed the hit song "Officially Missing You" by Tamia, where 'Play Guy's husky tone beautifully complemented 'Enter Lady's pure vocal color, creating quite the lovely stage. 'Enter Lady' also impressed by taking over the rap verse.

At the end, 'Play Guy' ended up winning the audience's vote by a slim 52-47, advancing to the second round of the competition. As for 'Enter Lady,' she revealed herself to be none other than LOONA's Heejin as she moved into a reveal stage performance of Sunmi's "Full Moon."

After the reveal performance, Heejin thanked MBC for the opportunity to return to the program as a contestant, stating: "I was happy and grateful to appear on the program as a panelist as well last year. This time around, I was able to appear as a singer. I couldn't believe it at first. There's a lot of pressure, so I was really nervous."

She then stated that her father had objected her dream to become a singer, so she never used to perform in front of others often, and wanted to be able to give a proper performance while appearing on 'Mask King.'

Meanwhile, Heejin and the rest of LOONA are currently promoting their comeback single "Butterfly."

Check out Heejin's performance and reveal stages here!