Here are some examples of urdu words that mean girlfriend (mehbooba,aashiqa, sanam). But all of these are too poetic....and too filmi....and really are not used in daily coloquial urdu conversations. You can call her your "heroine"......as they do in the Indian movies. LOOOOOOL, I crack me up! No, seriously dude, don't do that!
Why are you so interested in this question? Is it simply out of curiosity or do you actually have a "saheli" whose more than just a saheli?