I've often considered outlining characters using a GURPS character sheet, possibly throwing in Nature/Demeanor from the Storyteller system (White Wolf/Onyx Path). Pay particular attention to the character's Advantages/Disadvantages and Quirks/Perks as these will often give you some idea of the character's personality. That's also where the Nature/Demeanor thing comes in. Nature is the core of who the character is. Demeanor is the mask that the character presents to the world. They can be quite different, and the dichotomy can be a source of internal conflict if you want to go that way.
Here's an alternative character outline scheme.
Here's an alternative character outline scheme.