The term "supporting character" is a broad category including any character that, as the name implies, somehow supports but does not carry the story. Many people interpret this term differently, claiming that a supporting character must support the main characters. This is true insofar as providing a sounding board for the main characters is a form of support, but the true mission of any character of this type is to add depth and interest to the plot and story overall. Supporting characters appear in many different types of media, including television, written stories, and even video games.
Broadly speaking, a supporting character is a character with a personality and some degree of permanence in the story but whose choices and actions are not the primary focus of the story. Supporting characters may have their own missions or may be the focus of episodes, as is the case in many television series. Even so, they are not the characters on whom the audience depends for a story to be complete.
There are several categories into which a supporting character may fall. These can be used to help further identify the role the character plays in a story. Not all supporting characters are positive influences or allies, nor must they necessarily have a relationship with the protagonist. Some supporting characters never meet the protagonist and may be allied with a negative character. Negative characters are sometimes called foils.
Depending on the type of story, the role of the supporting character in the story may be quite different. This character might give the protagonist something to do or might provide a partner for dialogue. Even though these characters have very different roles, one characteristic of good storytelling is that supporting characters are easily distinguishable and have strong, fully realized personalities.
Some types of stories have many supporting characters who may change frequently because they are not essential to the continued plot. This is true of many television series, where the necessity of keeping an actor on a program can cause problems. Supporting characters are not always vitally important to the continuation of a series, but loss of a character can cause problems for storytellers.
In some cases, supporting characters can grow more important than the main characters in a story. This is particularly true of ongoing programs, such as television shows or comics. When this happens, a spin-off series can be created in order to turn a supporting character into a main character. It is not very common for former supporting characters to become the main characters in the show in which they were created, although this is possible with large ensemble casts.