Who is that security guard?

During the movie, he shows up sometimes (year 2000). He seems to know a thing or two about radios. In a scene where In is talking to a guy friend and inquiring where Su-Eun is, you can see that security guard stop by behind them and then smiles and leaves. He also told In to go home because thats how time passes by quicker. Was he a character from Su-Eun's time period?

In a scene where Su-Eun realized that she was still clutching the HAM radio, she goes back to the classroom to return the radio but the door was locked. As she looked down the hallway she sees the janitor/security guard walking away twirling a set of keys. Was that the guy that we keep seeing in In's time period?