Means that Mary is another girl (we don't know who), not the one you pointed. It also could mean that Mary transformed herself somehow (looks different or acts different)
My point is that "Mary is other girl" is incorrect English and is not said. It has to be "Mary is another girl".
Yes, you're right, but I understood the josip02's question and told her/him how do you say "Mary is another girl". Obviously, josip02 is not a native English speaker. Me neither.