"Stalker" is an unnamed indivdual who only appears for a single scene, in which she purchases a large amount of Ink Bombs. It's implied that "Stalker" is the swimmer that Cassandra accidentally changed gender on by throwing a gender change potion into the Lake. In that case, "Stalker" is in reality a man that got his gender changed to female.
"Stalker" has attempted to attack the Protagonist three times as of version 0.37, but all of them failed for various reasons.
During the third attempt, if you choose to pull the rope, it'll land on an unknown individual, hinted at to be "Stalker".
Stalker is shown to have quite a short temper when encountered in the Item Shop. At one point, "Stalker" says "Wait until I get my bo---" but is cut off upon seeing the Protagonist enter, after which "Stalker" hurridely leaves.