For a while now I have maintained that a person has the ability to chose at any moment how to react with the circumstances of that moment. Personality, as a whole, is an intentional construct; a person can chose to behave one way or another throughout life, and an audience can evaluate that person either favorably or negatively. This is oversimplifying the matter, but worth noting is that personality is at least partially a cognitive process, and by being such, is affected by the process of cognition.
At any time, the way our thoughts appear in our heads are organized by the system which our memory recalls values. Memory, being a highly flexible (and somewhat unreliable) part of our cognitive process, can load various values at different times. For example, take person F who at times is very calm, and at others highly irritable. When F is very calm, F's decisions are perhaps regulated by the value of tolerance, which memory is upholding throughout F's cognition in response to external stimuli. When F is highly irritable, perhaps the values of tolerance are not brought fourth to a large degree by memory, and instead some other values such as selfishness and loathing are predominant in F's cognition in response to similar external stimuli. F is not a "different" person in either case, and it is generally accepted that F has some degree of choice over how tolerant the response should be.
In a hasty conclusion: Who am I right now? I am the values I have managed to remember.