Category five (stemming from the term used to describe hurricanes) refers to a man who is a category five stalker. This man will stay obscenely close to a girl a bar in attempts to get her to notice him. He may also follow a girl around, or show up in places where he knows the girl may be, such as her house.
Category five can also refer to a man who is category five crazy, or category five creepy.
Sarah warned Jessica that she had a Category five staring at her from across the bar.
April knew he crossed the line to Category five when she found him waiting in his car in the alley behind her house.