Heroes are applauded. Main characters are beloved. Secondary/tertiary characters move the plot. Without these characters there is no story. They are the ones who carry the load, chapter to chapter, the bearers of good and bad news, and in truth, heroes and main characters are nothing without them. They support the main character throughout. Have you ever liked a secondary character more than the main? Are tertiary characters disposable? Do they serve a purpose, fill a hole, and are forgotten after their purpose is fulfilled? Are they neglected characters, in a sense? I don’t think so. Inside every tertiary character, however short lived, there is motive, truth, and purpose. It is that last piece of the puzzle, the one that completes the whole picture. Their existence is brief but not without meaning. After all, they appeared in the writer’s mind for a reason. Even if they are quickly out of mind and out of sight, they became part of a chain of events that if broken, disturbs the scene/story.
In these characters defense, they work for their keep, and unlike main characters/heroes, they are not handed a crown from the beginning.