When writing you just cannot keep all the different characters personality traits in your head, if you do you might end up with a character that gets out of control and that is not a good thing.Developing characters that do what you tell them to do.
A character outline is sort of a record of a characters personality, it can help you stay consistent when dealing with multiple characters, in the writers mind you have to be able to put on a characters hat so to speak.
First begin with the name.
Then get used to referring to that character in the way that you describe that person, be it a male or a female. Imagine how they look then write down that description, avoid going over board, with descriptors.
Keep it simple but precise.
Characters have personality in fact many authors claim to have experienced characters that just seem to have a life of their own. Defining your characters in outlines is a great way to keep up with how your characters behave and think.
Describe your character.