What does it mean when you have a dream and a girl appears and says you will know me soon?

The most likely interpretation is that this is a wish-fulfillment dream of a guy who would like to get to know a girl. Alternatively, the girl symbolizes a neglected or suppressed part of the dreamer's personality that is about to become active in the dreamer's life. It could be one of many different things, from creativity to a new interest to blossoming sexuality.