See how often they participate in the script: HERE.
Character Story: The Postman is a recurring character through the Zelda games. He has a tendency to appear out of thin air to deliver a letter to Link.
Race Story: Humans typically live peacefully with other races throughout all of Hyrule. The more specific Hylian race is a subcategory of humans. They can be differentiated by their ears -- Humans have rounded ears and Hylians have pointed ears.