"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". If you choose not to pull it, you can "almost hear what sounds like a female voice cursing in the distance as you walk away from the sign", further implying that "Stalker" is the one out to get you.
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" quickly leaves.
In terms of looks, "Stalker" is an ebony character that has black hair reaching down to the neck and is dressed in a green tank top and shorts.