When it comes to writing characters, any and all of them can make or break your story. Everything that happens, or doesn't happen, revolves around them.
A book is a rich tapestry of many different stories, woven together to make a picture worth hanging on the wall in the High King's castle. Every character is important, not just the main character(s). No tale is complete without a compelling villain, side-kick or love interest!
There are many details to consider for all of your characters. They should be engaging to the reader and have multiple reasons for acting the way they do. They should be realistic and flawed. This will make your characters believable and help your readers relate to them - even if they're a two-headed, purple elf from deep in the ocean.
Don't fall into the trap of having a Mary Sue*, or her male counterpart - Gary Stu!
* From Wikipedia: A Mary Sue is an idealized and seemingly perfect fictional character. Often, this character is recognized as an author insert or wish fulfillment. They can usually perform better at tasks than should be possible given the amount of training or experience.
A good example of a well-rounded character is T'Challa from 'Black Panther'. He allows himself to show emotion, he gets angry and he gets confused. He also loves deeply, makes jokes, is fiercely loyal, and is a talented fighter. Viewers might not have the technology or power (political and super) that he does, but it is easy to relate to his feelings of insecurity and the conflicts he has to overcome. He realises that people he admired are not perfect, and that the world is changing. This is something that everyone faces.
Providing a detailed backstory for your characters will ultimately give the reader a better chance at becoming attached to them, whether all of these details are revealed or not. This isn't to say you need to create entire languages, like Tolkien, or encyclopedic information that will never see the light of day like Rowling.
In this section we will explore different aspects of character development. We'll look at ways to bring your main character to life, as well as some of the possible motivations for your dastardly villain. We'll discuss the importance of diversity, tired tropes, and how to ensure each character has an individual voice to go with them.