Actresses Shin Si Ah and Han Yezee expressed their support for Kang You Seok's debut on 'M! Countdown.' On May 22, the official social media of a tvN drama shared a picture of Kang You Seok with the two actresses making heart gestures. The post asked, "Who came to cheer for Um Jae Il on 'M! Countdown'?"

It was later confirmed that the two actresses were Shin Si Ah and Han Yezee. Another post revealed a photo of the trio posing in front of the artist waiting room sign for Um Jae Il. The caption mentioned that Pyo Nam Kyung and Kim Sa Bi rushed to support Um Jae Il (HI-BOYZ, Yulje Hospital), which was a real occurrence.

These three actors appeared as first-year residents in the Obstetrics and Gynecology department at Yulje Hospital in the tvN drama 'Resident Playbook,' playing characters who were close colleagues and friends.

The group HI-BOYZ, featuring the character Um Jae Il portrayed by Kang You Seok, originated from 'Resident Playbook.' Their debut single, 'When the Day Comes,' became popular with its story of young love set to lively guitar melodies. Kang You Seok garnered attention for a performance video with TXT members Soobin and Yeonjun.

'When the Day Comes' quickly entered Melon's Top 100 chart, and the teaser of the performance video became a trending hit on YouTube. Riding on this success, Kang You Seok appeared with HI-BOYZ on 'Mnet's M! Countdown' on May 22.