
Right I was confused about that random coworker introduction too... turns out there was a skipped chapter between 11 & 12 (i.e.: the epilogue should be the 36th chapter). Spoilers for the missing chapter follow (I dunno how to upload here)...
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Basically, it's about the "field project" (business trip) that Hyunsung took on, at the end of chapter 11.
It starts off with Hyunsung (HS) trying to find his way to the office building he was sent to. He accidentally bumps into a guy (G) on the street; a second later, after exchanging sorries, G stops him to return a handkerchief HS had dropped.
HS eventually makes it to his destination, and finds out that G works there. Their exchange here seems friendly enough. There is another scene where G "accidentally" bumps into HS again, and laughs it off. This happens several more times, with G keeping it pretty light-hearted. At this point, HS is a bit annoyed.
Finally, it shows HS in line at a cafe, where G is right behind him. G cheerily tells HS, "You like yogurt right Hyunsung? Maybe I'll try one." HS is understandably a bit skeeved, and wonders how G knows so much about him... if G is actually his stalker? HS then laments that he's become over-sensitive.
They are now back in the office, and HS is busy with some paperwork. G playfully scares him ("boo", shoulder grab), and a shocked HS asks him what's up. G invites him out for some tea.
Scene cut to a cafe, where they are sitting in a booth with drinks. G says that he's sure HS is surprised about how he knows so much about him. He reveals that HS's "friend" came by and asked him to "look out" for HS, and told him all of HS's likes/dislikes. Apparently G did not believe that "friend" was for real, since it's a bit weird for someone to ask something like that of a colleague (no less a colleague on a business trip). G was surprised when everything the "friend" told him about HS turned out to be true: food preferences, habits, penchant for coming to work with hair sticking up (lol). So G initially told himself that this was probably just an old friend who's a bit protective; now that he sees HS's off reaction to this info though, something seems not quite right. G asks HS who the guy is, and if he's after him for money or something.
HS reveals that he is the object of a stalker's obsession.

No problem, it was bugging me too, haha.
I know, it was a plethora of plot holes... like first of all, how did the stalker even know who worked at the company HS had to travel to? How did he know this guy wouldn't be weirded out by him, and derail his whole plan? It was also a big coincidence that the guy himself was quite iffy, with the beer/soju switch (speaking of that, how did HS not taste the difference or notice the color change?)
Anyway, long story short: I agree, lots of things didn't make sense
I honestly had a really good time reading this. I really enjoyed it. The part I thought was the most interesting was the whole arc with the stalker tied to the chair. I felt it revealed interesting psychology for both characters.
There are still questions left over. I guess it's not necessary, but I'd like to know how they even met, why the stalker was moving apartments, how was the living state of his old one? Why did he decided to start stalking people, how long has he been at it, & what kind of life drove him to become that? Maybe all those unhanded questions are what keep a mysterious feeling.
In addition, the translation was difficult to understand at certain times. I also feel there is a chapter missing between 11 & 12 when they introduce the co-worker who tries to help the mc, only to get threatened. He's thrown in there so abruptly.
Those elements added confusion, but other than that, it was a great read.