It was Taegu at the door~
The panel after the visitor turns to leave shows Yeongwoon's bag in the center... this is a deliberate clue to the identity of the visitor.
When Seo In drags Yeongwoon away from Taegu's car in the hospital parking lot, he doesn't have his bag on him and continues not to have it while on his date with Seo In.
After watching the movie in chapter 56 and then going to grab food, Yeongwoon says to Seo In, "Sorry, I left my bag in Taegu hyung's car, I'll pay you back later."
Then, the bag magically appears after the visitor shows up...
It's definitely Taegu.
Side note: In chapter 54 and 55, Taegu is wearing jeans, black socks and those white sneakers shown in the final panels of chapter 57
Who tf was at the door???? Imma throw hands ts is pmo someone's always gotta ruin gay sexy fun times >:(