A character who is never met in person but is only talked to in letters?

What is it called when there is a character who is never met in person. The main character writes letters to this character . The letters that the main character write are never responded to, because she never mails them, but the person she is writing them to does exist. What is this type of character called? The book that has this kind of character is Parvana's Journey, by Deborah Ellis.