Bet the stalker is actually some other dood(we’ve probably seen) and this is a set up by the real stalker. Gonna be a classic gotcha bitch moment, a bait and switch, Like the old school gravel traps in Minecraft.
@steebus Actually I think the stalker is the girl the senior harassed before, but is now friends with the big sister. She seems to like the senior with the blonde hair, who in turn likes the big sister so I think she's jealous. She may be using the stalker stuff to take out her anger on her, or maybe she was hoping she would get scared and hole up so she'd be out of the way. I think at the festival we also saw her putting something in the big sister's bag.
Even though they're that old, getting into a romantic relationship is a first for them. Well technically this is Han-Eol's second relationship but he was coerced into his first one without his knowledge. Because of the incidents in high-school they didn't care to explore those feelings later on in life.