It’s KHN, baby!
The actor who slayed in Scarlet Heart appeared as himself in this cameo, as an actor dating Ahn So Hee on Entourage [Episode 5]. Or, rather, they had dated, and he wanted to get back together. This creates a predicament as he was interested in getting cast in War Terminator with her, and thus also, would be working with Cha Young Bin (Seo Kang Joon).
And Cha Young Bin, one of our leads, likes So Hee, and finds KHN in her dressing room.
Nice to see you, Kang Ha Neul. I’ll take a cameo if that’s all I can get.