Mirrors in fiction are characters who reflect the main character and reveal insights into the main characters behaviors, thoughts, and motivations. A mirror allows the main character to check the state of their own being or learn lessons from their mistakes.
Secondary characters often act as mirrors for main characters, however, that should not be their only purpose in a story.
Character traits in a secondary character that will be used for a mirroring effect need to be used realistically. This means that these traits need to come from somewhere and not exist in a vacuum.
Develop an element of his or her backstory that explains the trait, such as relentless pursuit of monetary success being related to a parent who could never consistently provide for that character as a child. The main character can see the actions the secondary character takes to reach his or her goal and use that as a barometer for how much she is letting her drive for financial success compromise her moral standing.
This level of similarity is recognized by the main character and causes dislike or disgust (or with an opposing trait, longing and need for a similar situation like a happy and respectful relationship). The main character thinks he or she knows the secondary character and can predict or expect a certain action or reaction, but often fails to fully see the same in his or her self, which leads to making mistakes. The reader sees the bigger picture and similarities more fully when this is done well.
Having said that, if the secondary character only exists as a mirror, that character will feel flat and less relatable. If their actions or emotional responses are too easy to predict the reader will be bored and likely not find that secondary character believable, which then lessens the mirroring effect. Emotions and drives are complex, and that needs to be true for secondary characters as well as main characters.
Well developed secondary characters are not always predictable, but when they do act as complex mirrors they provide opportunities for the main character to make mistakes and learn from them in a more believable way.
While secondary characters are typically less integral to a story, that doesn’t mean they should be any less real.
Secondary characters should still have some level of a character arc, though it will necessarily be less complex than the main character’s arc.
Even though much of a secondary character’s backstory may never see page time, it’s important that you know their story and personality so you can weave it into their interactions with the main characters. This will not only make them more memorable, it will also make the interactions more meaningful and provide a fuller scene with the main character.
Secondary characters should also have a purpose. They should, in some way, help to progress the storyline through their presence or interactions with the main characters. This may come in the form of providing assistance, revealing information, being an emotional support, etc. How a secondary character progresses the story depends greatly on the type of story it is, but be mindful that they should be serving a purpose and not be acting a page filler or a basic sounding board.
It’s also important to consider a secondary character’s motivation. Why is he or she helping this character? Because you, as the author, need them to is not a good enough reason. Reach into their backstory to find and then develop their motivation. Help doesn’t have to be entirely altruistic either. The secondary character should have mixed feelings about providing help, or be reluctant to offer it. It could also be self-serving or their presence could act as a hindrance or barrier rather than being helpful.
Secondary characters exist on either side of the protagonist-antagonist spectrum. Don’t short change secondary characters involved with the villain/antagonist. These characters can be important cogs in the overall mechanism of how the antagonist’s path and ultimate resolution unfold. An antagonist taking something too far could prompt a secondary character to walk away, leading to him or her reconsidering what they are doing. Alternately, a secondary character who wants the antagonist to fail could give the final nudge to push him or her over the edge and initiate a chain of events that leads to his or her downfall.
Secondary characters may have less page time than main characters, but their influence on other characters and events can be profound when the time is taken to full develop them and integrate them into the story.