We all know that a main character is essential to a story. We need their viewpoint. Sometimes there are multiple MCs. We usually surround these viewpoint characters with friends, family, and opposition characters. Heroes, Helpers, and Antagonists are all commonly spoken of.
But what about the characters we never see?
There are several reasons for having a character that never gets any face-time in the story realm. The most prevalent of these reasons is that the character is from history and is no longer living—this can be an ancestor, old hero, ancient foe, or random uncle to whose estate your MC is suddenly made the heir to.
(Radagast the Brown, who earns a face in the movie An Unexpected Journey.)
In Lewis’s Prince Caspian, King Caspian IX is a relevant and extremely interesting character.
Imagine these stories without those characters.
Andrew Peterson’s On the Edge of the Dark Sea of Darkness is filled with mentions of interesting people we never meet, some important and some decidedly less so, but none less remarkable for all that they are invisible.
Without the explanation of that little bit of wildness in the Took clan, we couldn't explain Bilbo suddenly feeling the urge to run off on a grand adventure. Without the great and regal history of the dwarvish kings, who cares if they get their mountain back? With the knowledge of the elder Caspian invading Narnia and all that he did and what his character was, Lewis displays for us a picture of what the old king's son might be. And without all the literary ghosts of Oskar's past, well...Oskar simply wouldn't be Oskar.
Characters who are never actually seen in the tale can be absolutely vital and color the story in ways the characters at the forefront cannot. They lend a depth to the tale that makes it feel real.
This character type has a lot in common with Scenery Characters, which I’ve already devoted a long post to. (You might be able to tell that the more detailed aspects of casting have lately been on my mind.) But they are also in a class entirely of their own. After all, some of these characters might be the most important bit in the entire story. The pirate that buried the treasure. The inventor that inspired a success. The hero that dictated a quest.
Try employing such characters into your story world—whether fantastical or modern: People good and bad, important and minor, seen and invisible. ‘Historical’ or faceless characters add interest to your story and often you don’t have to spend a great deal of time developing them, while they can be great for developing the characters which stand in front of them and are at the focus of the tale.
As a writing exercise, be on the lookout for different types of characters, particularly minor characters, in the stories you read and the movies you watch. Try to label them and find their significance: What do they add to the story being told? What would be lost or gained without their presence?
Use what you observe to aid in your writing, helping you to cast the perfect guest-list of your tale.