I don't think that there is any real "correct" answer to this question. If you really think about it, this should really be the easiest question that an interviewer could ever ask a prospective student.
Like dpark74 said, there is nobody who knows the answer to this other than yourself. I think that people tend to be insecure with this question because they feel that the interviewer might perceive you to be unfit to enter medical school, yet all this question is asking is to describe yourself. Your grades, MCAT scores, and LOR's determine whether or not you will be able to handle medical school, not this question.
As long as you don't answer by saying "I am a serial killer and I'm wanted in 20 countries," I'm sure that whatever you say to answer this question will only help you. If anything, this question just serves to see if you are secure with the person that you are. If you are fumbling and stumbling when you answer, it might me a sign that you really don't know yourself or are too insecure with yourself that you are afraid to be truthful. Let's face it, if you get far enough to be invited for an interview, they already like something about you.
I simply suggest that you write down a list of things that you think others see you as and you think you are, and put it into words that would adequately answer this quite simple question.