Personally, while I do like making a fully-fledged character sheet, if I may, I'd like to propose a few challenges instead:
1) An exercise in breviloquence: Describe succinctly each character in one sentence.
Seems simple enough, right? But conciseness can be difficult achieve, if there's nuanced details to cover.
2) A declaration of inspiration: Describe the dreams each character and how that influences what future they are striving for.
What matters in a story is not the "who", not the "when", not the "where", but the "why". Thus, illustrating this "why" factor is desperately necessary.
I believe covering these two would really be sufficient to capture the essence of a character. And, of course, we can add the basic demographic information--name, age, and so on.
Other miscellany can be uncovered later as we see fit, either in-character or out-of-character.
Thoughts?