I am not sure what people are getting at with this 'don't lie' about a girlfriend. If you are coming to visit a friend, you are coming to visit a friend. How you personally define your level of intimacy with that friend is personal and irrelevant. There are no laws that specifically define when one friend becomes more special than another until that person becomes your fiancee.

Also, while you should be prepared to present proof of your intent to return to Germany (return ticket, employment, etc), you should not present it unless you are specifically requested to do so. If I were an immigration agent, I would really suspect someone who, without being asked, provided me all this documentation -- sort of a 'methinks he doth protest too much -- moment.