"No, you are my partner."
Translation:Nein, Du bist mein Partner.
It depends on case and gender of the noun. In this case, "Partner" is masculine, and the case is nominative, so it is mein.
Also, take note that "du" no longer needs to be capitalized but you may come across it spelled that way in older texts. http://german.about.com/library/weekly/aa020919b.htm