The theory of personality that best captures normal variation of human behavior is psychoanalytic theory advanced by Sigmund Freud. Psychoanalytic theory is built upon the conscious and unconscious mind. The conscious mind is what you are aware of at a given moment, your present perceptions, memories, thoughts, fantasies and feelings. Then there is the preconscious mind or what we can call available memory. Things we can easily bring to the conscious memory if need be. Then there is the unconscious. The unconscious is the largest and most important. It is defined our drives or instincts and our past experiences that cannot be brought to memory. This theory best captures the normal variation of human behavior because it is individualistic. It is also built upon the unconscious which is impossible to control and hence behavior cannot be faked. The theory is holistic in that it explains the sum total of who we are and the experiences that shape people into the different personalities that they are. The uniqueness of every individual depending on the life experiences that person has been through and socialization.
Personality is not a set of fixed traits that you demonstrate consistently across life but rather it is tendencies that change across time and with experiences. Personality is shaped by our life experiences. Experiences that persist for a longtime can actually change a personality. Therefore personality is flexible and dynamic that that keeps on changing across the lifespan and is shaped by experience. For example a child brought up under abusive parents may become violent in future but if that person finds people to love him, his character will change. Equally children who suffer domestic violence will all get depressed. But if some realize their condition and what they can do to change it, they will overcome that depression and be different. Personality therefore changes.