There are a number of traits you can assign to your character or roll randomly on page 29 in the Core rulebook for them in each of three different areas:  personality, motivation, and alignment.  You may make as few or as many of these rolls as you choose if you do not have a predetermined idea for your character.  It is, however, recommended you make at least one roll in each area if you do not already know your character's personality or you want to play something different and random.