This is a very interesting question because it can be looked at in so many different ways.
I personally have always hated these kinds of questions, because it’s very hard to define yourself. When you define yourself, you’re saying this is me and that’s that. What really I really think defines you though are your actions, and it’s hard to sum up a lifetime of actions into a definition of a person. It’s always interesting to hear what people say though, because it shows a lot about their personality and what they find important, which I suppose is the point of the question. I think that this kind of question is just designed as a shortcut for getting to know someone though. I think that getting to know a person is really the only way to know what defines them. They could tell you that they’re a “nice person”, but what one person thinks of as being nice might not line up with your opinion. Seeing their actions is really the only way to know what someone’s really like.