I bet you anything it's the professor because like in one panel of the chapters it shows the teacher in the car from afar watching Joseph ╮( ̄▽ ̄)╭ mingyuXwonwoo
I heard from the spoiler that there are actually 2 stalkers. Now that's a twist but i don't know who but i think one of them is the teacher so maybe there's a possible that glen also a stalker or maybe a new character or a side character that we have missed in the first few chapters ╮( ̄▽ ̄)╭
I think the culprit is Glen? Cause why else would the manhwa be named after him?