quite a good question in a way. Is it a card that can describe someone or is it where it appears and the spread you use? Possibly both can apply. I know court cards have already been mentioned, and they are a good starter for 10. Got be be careful with them though when describing physical attributes, particularly if the female (or male come to that) is like me and changes their hair colour with regularity!!
Many years ago when I was taught the tarot, my mentor showed me a spread he had used for years. His belief was, if he described the sitter fairly and accurately with this spread, it bode well for the main reading. If he was ever inaccurate, he would offer to suspend the reading there and then with no cost or comeback to the sitter. In effect it was a litmus test and set the scene for what was to follow, as well as being an indication as to how well he was tuned for that reading etc.
I have used it many, many times since, and if I am reading for someone for the first time it is always offered as an opener. There are variations on this I am sure, but it has been one that helps me understand the sitter before then looking at how their personality or traits may have a bearing on the main reading.
I describe a person's personality through using a 7 card spread as follows:
card 1 - how they come across to people outside their close circle. (l.e. colleagues, acquaintances etc).
card 2 - their main energy within a relationship, or what is happening with their relationship
card 3 - what they are feeling or showing emotionally at this time
4 - what they want from life in general, what would make them happy
5 - how people would describe them who are close to them (i.e. family, close friends)
6 - where they are now
7 - spiritual outlook
I have written a variation of this which includes personality traits in more detail, but this one is the one I come back to - it has worked for 12 years now, so tend to stick with it I guess