Louie was a resident of the building and the inhabitant of apartment 4B before Penny moved in. They were originally mentioned in the pilot episode, being described as a "200 pound transvestite with a skin condition". They were mentioned again in the second episode of the series, "The Big Bran Hypothesis". Sheldon states that Louie's closet was "immaculate", keeping their dresses, cocktail gowns and police uniforms in order. Sheldon had helped them install a webcam in their bedroom.
The only time Louie appears in person is in a flashback scene in the episode "The Staircase Implementation" (S03E22), when Leonard accidentally knocks on the wrong door when responding to Sheldon's advertisement for a roommate. Louie opens the door wearing a pink full length dress and diamond earrings telling Leonard that he has the wrong apartment and wants the "crazy guy across the hall".