Apparently my left eye and left nostril don't like dust/dirt because even though I did all that yesterday they are both still giving me trouble. For anyone who was wondering, no, allergies are NOT fun. I hate allergies. I also hate sneezing, but I digress.
Todd Stone calls the friend, or supporting characters, "window characters" because they give us a glimpse into the inside of the main character(s) personality--perhaps not something that the character reveals on his or her own.
These "window" characters are sometimes sidekicks, friends, colleagues, you name it.
Think about it. You have these types of characters in your own life. Maybe it's your spouse...the one who says "You know, you ALWAYS say that." or "You are such a computer addict." And you think "No I don't." then when reviewing yourself, you find out, that yes, in fact, you do.
Now whether or not your supporting characters bring up some incredible "Oh my gosh!" revelation in your main character(s) lives/personalities is completely up to you and how you write the story. Maybe they're simply there to act as a voice of reason or wisdom. Or maybe they're there to offer a sense of stability if the character(s)' life is being turned upside down.
Look at Shakespeare's Romeo And Juliet. Benevolio is Romeo's best friend (along with Mercutio) and the two of them hang out and encourage him. How about Juliet's Nurse? She's Juliet's closest confidant. (And you have to love the Meagan Follows version with how the actress who played the Nurse says "Scurvy knave!"; Follows plays Juliet and she does a very good job).
Maybe you take a look at Charles Dickens' A Christmas Carol each of the 3 ghosts reveals something about Scrooge, either how he became such a miser or parts of him that showed he had the capacity to love.
Have you seen the musical, 1776? Abagail Adams reveals truths about John Adams characters in their duets and "talks" (representing them writing back and forth to each other as they did during the time he was away...they were incredibly close--even history books say so--and relied on each other for support, encouragement and advice). J. Adams was a firecracker, but his wife was one in her own right, as well.
Whatever you do, whether the supporting cast is revealing things about your main character(s) or offering them any help they need, they can be just as important to your story's plot as the main characters themselves.
Have A Fun Friday!