I always find that the most potent way of creating a character is to give them one (or both) of these two things:
+ A powerful need, want or goal;
+ Something they desperately wish to avoid.
My way of writing involves creating the characters before I have any real idea of the plot. Character creation first allows possible plot threads and ideas to evolve directly out of the needs of the characters--which, if you think about it, is how it should be. Plot revolves around characters.
Once you've figured out the big question, you can move onto other things. These may be necessities for that particular character (eg. must be of a particular age, must be able to do certain things), or they may be more general.
For more information, I highly recommend Holly Lisle's articles, located at:
http://hollylisle.com/fm/