In non-fiction books, supporting topics can be useful to help give credence to your topic or to enhance it in some way. These topics should, of course, have a strong connection to your main topic. For example, if you are writing about how to grow a container garden, it can be helpful to also let your readers know about the benefits of eating fresh produce free from chemicals and hormones or about the environmental benefits gained from growing your own produce. Or if you are writing a book with instructions for weight lifting properly, it can be good to also write about how a healthy diet can enhance fitness levels or about the way our modern lifestyles have changed the way we commence exercise since the days of homesteading in the 1800s.
In fiction novels, supporting characters can be anyone. A supporting character can be a parent, a friend, a sibling, an aunt or uncle, a cousin, a neighbor, a pet or even someone brought into your protagonists life through the events of the story. The supporting character can have a personality and traits that are similar to your main character, but it usually works best when their traits are different. Those personality and physical traits can compliment your protagonist though.
Supporting characters are an important way to show off more of your main character. Through dialogue and interactions between your protagonist and your story's supporting characters, the reader can learn more about your protagonist and find even more reasons to become invested in the outcome of the novel. The way your protagonist interacts with the supporting characters and reacts to their actions becomes a way to display more of his or her personality, strengths and weaknesses.
Your novel's supporting characters can help to move the plot along. For example, in one of my stories, the main character has a best friend who is trying to help her survive the threats from one of the antagonists in the story, but, when the protagonist thinks she might have found a solution, the antagonist turns his evil intentions toward the protagonist's best friend. In this way, the tension is increased and the task before the protagonist becomes even more treacherous for her to navigate. Another example of a supporting character helping to move the plot along can be when the protagonist is an amateur detective and his or her friend or parent is a police officer. The police officer can impart information that is vital to your protagonist's investigation.
When naming your supporting characters, it is best to begin their names with a different letter than the one with which your main character's name begins. You don't want similar sounding names, because this can sometimes be confusing to the reader. I once read a story that had three characters with names that began with the letter A. They all had the same syllables within their names too. The book had many characters, and I found it difficult to keep track of which one I was reading about whenever one of their names turned up in the book.
Normally, it is not a good idea to write too many supporting characters into your novel, unless your novel is an epic fantasy that will progress over many years and several books. As in the problem with naming character,s this is because it can become confusing to your readers. Sometimes, the reader won't know whether to get invested in a certain supporting character or not when there are too many supporting characters.