Often people have a persona that covers their true self. Take that idea of a mask and apply it to a character. Design them with a strong personality, but beneath it have a second sense of self that is kept hidden. As you design these two sides (interior true self and exterior mask), try to have a few aspects cross between the two. Both prefer to listen rather than speak first or both like the color blue. The interior escapes from inside when it fits the exterior persona.
So the tough guy that throws knives so well is crazy about practicing darts before bed.