Remember that I'm using "personality" rather loosely here (I prefer "identity" or "essence" but "personality" helps convey part of the sense I intend). Impulse control is an important part of brain chemistry; is that all a human being is? Depends on whom you ask, but if someone receives an injury that destroys the part of the brain handling impulse control do we distribute their assets to their heirs and certify the old person dead, and a new one living? The brain is disturbingly good at reassigning functions previously handled by damaged sections to different ones that are otherwise never involved with them. To the extent personalities do reflect our innate identities and essence, they do so via brains (interface one) operating bodies (interface two) amid surroundings (interface three) that others bodies (interface four) perceive as stimuli transmitted to their brains (interface five). How sure are you that that tells the whole story? Yes, damage to our brains and bodies can impare our perception and awareness of ourselves and the rest of reality, but that it does not follow that such damage alters our self any more than it alters reality.