Louie/Louise was a resident of the building and the inhabitant of apartment 4B before Penny. He was originally mentioned in the pilot episode, being described as a "200 pound transvestite with a skin condition". He was mentioned again in the second episode of the series called "The Big Bran Hypothesis". Sheldon states that Louie/Louise was "immaculate", keeping his dresses, cocktail gowns (and then his suggested profession) police uniforms in order.
The only time Louie/Louise appears is in a flashback scene in the episode "The Staircase Implementation" when Leonard accidentally knocks on the wrong door when responding to Sheldon's ad for a roommate. Louie/Louise opens the door and responds that Leonard wants the "crazy guy across the hall", signifying that a transvestite is not the most unusual person in the apartment building.