Why would a writer want to mirror a character? A mirror character can bring to light a specific characteristic you want to have the reader discover. Serring up one can find you showing rather than telling about a particular trait. You can also show something that can be helpful or destructive by mirroring. Since you don't want to destroy your main character in some manner by such as death or loss of something valuable like money or position, you can show the mirror character experiencing these things.