Lampshaded: "We never get to see Charlie much, now do we?"
Invoked: Charlie tries to visit Alice and Bob, but "Dylan the Villain" (actually an Anti-Hero) chases him off. All offscreen.
Exploited: Dylan wants Alice's friend Elaine to do an unspecified dirty deed. Knowing that Elaine has never met Charlie, Dylan bribes her with a photo of him. (The audience never gets to see the photo.)
Defied: Someone finally manages to force Charlie onto the screen.
Discussed: "Charlie's so mysterious, isn't he? I mean, we hardly ever see him!"
Conversed: "Who is Charlie, anyway? I'm dying to know what he looks like!"
Implied: The characters always refer to Charlie using pronouns (as in "I think he was the best friend I ever had,") in situations where no character who is shown would fit.
Plotted A Good Waste: The original plan was to have Charlie appear eventually, but the executives decide to make him an off-screen character.
Played For Laughs: Any attempt to show Charlie onscreen by the characters will end up ill-fated: He never accepts invitations to anyone's house (maybe he's been called into work), any photos of him will be very blurry, and if he is supposed to appear on television, the set will break down (or the power will go out) just before he appears.