The Stalker seems to be following a certain character throughout the episode, going as far as to follow the character all the way to the roof of a building, but once The Stalker gets to his destination, he simply says “Ah, finally back at my conveniently placed house on the rooftop.” and heads inside his house.

He is a minor character with 1-3 lines.

His voice is just a guy. You would expect him to sound like someone who's extremely creepy or something, but nope! Just a guy. A guy with a house on the roof of a building.