The relationships between your characters can be a powerful tool for reminding your readers what the core themes and objectives are in your story. They act as a reinforcement, getting into the subconscious of the reader and subtly guiding them down the right track. Without this link between relationships and themes it is possible for the reader to become distracted from the core themes, or to lose sight of them entirely.
For example, if your story revolves around a character who is trying to find the identity of her lost family, but every relationship involving her is a romantic one, your reader may become confused as to whether they are reading a story about identity or love. Not to say you can’t include both of those things, but there should still be a clear focus. If love is the character’s main objective, then her goal should be to find love, with the romantic relationships supporting that idea. If her goal is to find her family, the emphasis should be on the family-like relationships surrounding her.
An example of this done well is the TV series from CW, The Flash. In this show Barry Allen develops the power of super speed. As well as the limitless possibilities presented, Barry sees his powers as evidence of his father’s innocence and immediately his primary goal becomes freeing his father from prison.
Despite the many other plots and subplots within the show, the idea of freeing his father (and finding the person who really killed his mother) is Barry’s primary motivation for everything he does. This theme of family is emphasized by the two primary relationships in Barry’s life. His foster father, Joe, and his mentor, Doctor Wells. Both Joe and Wells take on fatherly roles in Barry’s life, representing the two sides of his personality. Joe is a cop and Barry’s moral compass, keeping him grounded to the real world.
Whereas Doctor Wells is a scientist and leads Barry down a path of discovery, pushing him to enhance his abilities.
Together, these two characters, along with Barry, encompass the story’s core themes of family, heroism, and truth. The fatherly nature of the relationships also provides a subtle but constant reminder that Barry’s true father is still in prison, awaiting rescue.
The show contains other well developed relationships too, such as Barry’s close friends, and his love interest, Iris, as well as the antagonistic “Man in Yellow”. But at all times, the primary focus is on Joe and Wells, hammering home those core ideals.
This does not mean you are limited to one theme and one kind of relationship. You should include any and all ideas and characters that you feel are beneficial to the story, just try to find some common ground between the two. Below is a resource that can help you with this: