The Ghost/Playing With

Basic Trope: A character is often mentioned or referred to, but never appears.

Straight: Alice and Bob mention their childhood friend Charlie at almost every episode, but he is never seen onscreen.
Exaggerated: Alice and Bob mention Charlie all the time, mostly about a certain Noodle Incident, his outrageous personality and antics, and some Noodle Implements thrown in. Yet Charlie never shows up at all.
Downplayed: Alice and Bob just casually mention Charlie several times.
Justified: Charlie just never gets the chance to stop by, or he's too busy.
Inverted: Charlie is seen at almost every episode, but Alice and Bob never mentions anything about him. They had never even mentioned his name; it was said by a waiter at a cafe.
Subverted: Charlie finally appears at one occasion.
Double Subverted: Only for Alice and Bob to say that it's his relative and that Charlie has important things to do, so he can't come by.
Parodied: Any attempt to show Charlie onscreen by the characters will end up ill-fated; such as a photo of him turning out to be very blurry, the television breaking down just a second before he appears on the screen, and so forth.
Averted: All characters ever mentioned appear at least once.
Lampshaded: "We never get to see Charlie much, now do we?"
Defied: Sometimes Charlie appears, sometimes he doesn't. It's an even pattern.
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!"