Who is The engineering student?

The engineering student is a man Young dates casually in Part I while he is living with Jaehee. His name is not given, but Young also calls him K3 because he drives a Kia K3. Young first mentions this boyfriend while recounting his memory of being called a gay slur by two male nurses at a medical clinic. Young laughs about it, but the engineering student is angry. Later, after Young breaks up with him, the engineering student comes to his and Jaehee's apartment in the middle of the night and begins shouting outside. He believes Young lives with his family and he is angrily attempting to "out" him. Jaehee takes him to a bar and calms him down.