Unknown Character/Playing With

Basic Trope: A character who drives the plot along without every actually appearing.

Straight: Bob the Big Bad was sent off on his first conquest by the advice of a random stranger, who's never shown.
Exaggerated: The same random stranger set both Bob, The Hero Alice and the Big Good Clair off on their respective destinies. It turns out that they time travelled back to do just that, with the series ending as Alice catches them activating their Time Machine. We never see them, hear them speak or get as much as a name or description.
Downplayed: Alice finds a sword lying around labelled "Property of [smudged name]". A prequel has the original owner as the main character.
Justified: It didn't really matter who made the suggestion or left the sword lying around, just that it was there. It's the major character's reactions to it that mattered.
Inverted: Dan is an incredibly detailed, deep character with a huge amount of history with everyone in the cast. He has no bearing on the plot.
Subverted: It turns out that the time traveller was Bob, ensuring his plan would be set in motion on his terms.
Double Subverted: However there was no Stable Time Loop. Whoever travelled back in time the first time was someone else who probably did everything by accident....
Parodied: The unknown character is "promoted" to The Ghost in Fanon and treated as if every example of this trope was them. "Danny Boy" is a Wholesome Crossdresser Master of Disguise (at least one of their "appearances" took place in a Lady Land, while another took place in the men's toilets) with the ability to teleport, many joking arguments take place over whether they're a he or a she and their Trademark Favorite Food is baked beans. This eventually becomes an Ascended Meme and Dan is introduced into the show.
Deconstructed: The protagonists wonder who it was and if they could make things right by tracking them down.
Reconstructed: Bob would have ended up as who he is anyway, it just so happened that the unknown character was around to set him off at that moment.
Zig Zagged: Some characters who do something minor are introduced, others do whatever they do entirely off screen.
Averted: We're shown who the person speaking to Bob is before they meet Bob.
Enforced: The show can't afford to hire any extras on their budget.
Lampshaded: "So who was it that gave him the idea?" "No idea, doesn't really matter."
Defied: Alice does extensive research to Bob's background and interviews this character.
Discussed: "I guess we'll never know who they were..."
Conversed: "So who was it who-" "No one important. Just shut up and let me watch the show."
Exploited: Bob takes pains to keep whoever inspired him's identity quiet so a Time Traveller can't prevent them meeting.
Played for Laughs: It's implied (as in the parodied entry) that all the off screen characters are the same guy.
Played for Drama: Alice must track down "Dan" so she can find out what Bob's Evil Plan is.

Whoever created this page left a link back to Unknown Character.