I think what defines a person is not necessarily something that others can see, although sometimes it is. To me, what defines a person is his inner character. Much of the time, a person's actions may be evidence of his character; but sometimes even actions can be misinterpreted by others who don't understand someone's motives.
Allowing for the occasional, misunderstood, motive for an "out-of-character" action, I think most of the time people of solid character demonstrate it through their actions.
We so often see or hear about people who do all kinds of questionable, illegal, or otherwise "bad" things;; but who are said to be "good people who made a lot of bad decisions". What that most often means is that the person in question is a likable or friendly person, or is someone who is not "evil". To me, though, what defines someone is that character, and a person of a solid character generally doesn't have a string of "mistakes" of the nature that call character into question.
To me, what defines a person is never about his appearance, wealth, intelligence, age, health, or anything else that would be considered "part of what a person happens to be". It's all about his character. The good thing about our character, though, is that we can choose to be of good character - which means that "what defines us" is, ourselves.