Jae hee is the stalker from their younger years that's the guy in the phone. I'm guessing they were working together the whole time once cheol left him after the outing.
The other guy minyoung almost stabbed with a pencil is just a random guy cheol was occasionally talking with
Hopefully this helps out
I believe that’s Jaehee. He was Cheol’s stalker since high school or middle school. Cheol never really had much of an interaction with him but he’s still obsessed with Cheol because Cheol saved him or something once. He definitely showed up when Cheol and Mi-young went on that amusement park date. He came out of nowhere and started talking to Cheol but ended up creeping him out instead so Cheol ran away, I think. He is the one who was orchestrating some stuff, like giving Cheol’s younger brother that photo of him and Mi-young. He’s been fighting with Mi-young over Cheol for awhile now.

Who’s that guy on the phone? Was it that guy from in the flashback?
- For some reason my previous comment got cut off and not all of it is showing.
- Please, help me remember who that is? Is it that guy who was blabbering his mouth and Min Min made after him with a pencil (was it?). Or no?
Idk, help.