Unknown Character

Not all characters are important to a story. However, those that aren't are normally not developed that well, while plot relevant characters are. On the other hand, sometimes it can go the other way, and minor, undeveloped characters (or a two dimensional main character) can set the plot in a new direction. This trope takes that to its logical conclusion.

This character has an effect on the plot; however, they're never introduced or named or possibly even shown in the background. This is usually the result of them being a Red Shirt or Unwitting Instigator of Doom in someone's backstory (so what they did or what happened to them is only relevent for how it influenced the character whose backstory they appeared in). However, in the most extreme examples, their existence may only be implied (for example, someone who left their MacGuffin or Emergency Weapon lying around for The Hero to find).

Note that while this character might be revealed and fleshed out later, there isn't usually any mystery about who they were; their role is fulfilled just fine by them being just another face in the crowd and they need not have any further effect on the story. The best way to identify a character as this trope is if they can only be referred to by their contribution to the plot and in the past tense, making it clear that they're little more than the reason something happened (e.g. "That guy who gave Bob his sword" or "That urchin who stole Alice's wallet when she was buying her Dead Little Sister's medicine"). Indeed, the only reason they exist is the fact that they did something that had to be done by someone, and in this case that someone was nobody important.

Compare The Ghost, who functions as any other character would (and might even be part of the main cast) but is simply never shown on screen (they can overlap; the main difference is that characters who fall under this trope don't have any characteristics, while the ghost can still be a fully fleshed out character), Posthumous Character for already dead characters who are still important to the plot, and the Featureless Protagonist, who can become this trope in sequals. A Badass Bystander will often become this if they don't appear subsequently and aren't given any characterisation beforehand. If they becoming a recurring character in later works or adaptations, they will often evolve into He Who Must Not Be Seen or The Ghost as a nod to their earlier characterisation (or lack thereof). Contrast Lower Deck Episode and Day in The Limelight for when less developed characters are fleshed out by the plot.

Examples of Unknown Character include:

Anime and Manga

In Ghost in the Shell: Stand Alone Complex, Aoi aka "The Laughing Man" reveals that he himself was arguably the second part in the eponymous "Stand Alone Complex" (an event where many people spontaneously start copying something which wasn't there in the first place) and the real "Laughing Man" was an unknown person whose email exposing the micromachines company's coverup was found by Aoi.

Literature

Hoid is a mysterious character who has appeared in every one of Brandon Sanderson's books - save the Wheel of Time, I think, since it's not his series - seldom named, but inevitably the cause of something that eventually turns out to be instrumental to resolving the plot. Even or especially if a story otherwise has no indication of being in the same universe any of his other books.
King Galbatorix from the Inheritance Cycle doesn't appear until the last book, yet he is a constant presence in the series.
Big Brother from Nineteen Eighty-Four is technically the Big Bad of the story; however, he's never shown in person and it's left up in the air as to whether he really exists in the first place. The same goes for the leader of The Brotherhood (where it's not even revealed whether the organisation he leads even really exists).
In The Malloreon, there is an unnamed character who stole the Sardion (MacGuffin of the series) and deposited it at its final resting place to be found by the heroes 300 years later, along with his remains.
In Rebecca, the main character is the second wife of the eponymous Rebecca's husband. She's compared unfavorably to Rebecca without ever being told anything about her by his staff. Nothing is revealed abut her as they figure she doesn't need to know, except that she died. In the end the protagonist learns more about Rebecca and gains the respect of the inhabitants by saving them from a fire.
- In the film adaptation the head maid refuses to accept her and stays behind to die in the fire.

Live Action Television

Played with in one episode of Doctor Who. The Doctor goes on a shuttle, and socializes with everyone in the cabin. Except the hostess. It gets lampshaded at the end when he realizes no one knew her name after she sacrifices herself to protect everyone from the Monster of the Week.

Newspaper Comics

The Little Red-Haired Girl in Peanuts, Charlie Brown's always offscreen, always silent, always unrequited crush. She was briefly shown and named Heather in one of the animated specials, but this is not canon. A 1990s strip showed her in silhouette, dancing with Snoopy. In the 2015 The Peanuts Movie, she is not only visible but engages Charlie Brown in a conversation; her name is given on the test score sheet as "Heather Wold", which incorporates the earlier noncanonical first name plus the last name of the real girl who inspired the character.

Theatre

Godot in Waiting for Godot drives the plot as the characters spend the entirety of the play...well, waiting for Godot.

Video Games

Bastion has a couple, the most obvious one being the unknown man who seduced and betrayed Zia, which lead to both her surviving the catastrophe...and to her father setting it off. Another example would also be whoever ended up with The Kid's money, which he'd been sending back to his mother (who was already dead). Forcing him to take another tour of duty as a Mason (although with the loss of his mother he might have done so anyway) and surviving the catastrophe.
The Rat Man in Portal who's scrawled graffiti all over the place (although he might be closer to The Ghost, given that he's essentially interacting with the story still). A comic given out with the game's sequal fleshes his character out.
In the game Singularity, you often come across hidden messages that seem to be addressed to you, specifically. The messages are from someone who seems to know you, and who also seems to have done the same things you're doing; before certain major plot points, the messages will actually give you the heads up before anything's actually evident (i.e. "DON'T TRUST HIM", etc). It's later implied to be a future version of yourself who went back in time to leave the messages.
In Quake 4, the protagonist from Quake II is this (Quake IV being the direct sequel to Quake II). He single-handedly invaded the Strogg homeworld and assassinated their leader, allowing a full-scale human invasion. He is never shown or mentioned by name.

Real Life

The soldier who fired the arrow which killed Harold Godwinson in the Battle of Hastings (thus changing the course of English history).