Personality is a function of entrenched behavioural patterns. Those behaviours and habits writ large in nerve to tissue connections and muscular development are those that become characteristic of the person and exhibited as their 'personality'. Because of the robustness of those patterns, personality as a set of characteristics is a relatively stable entity. We can rely on personality not to change much during the lifetime. Many pride themselves in being able to identify someone's personality from the moment of birth. Just as many others document rapid personality changes that happen between puberty and adolescence. Personality is a crucial means by which we identify people and through which we interact/enjoy/don't enjoy being with them.

Personality is tied in with core functionalities and any change in the latter is going to impact the former. In other words, functional change involves some change in personality characteristics.

While entrenched behaviour patterns are stable, ie: they are hard to change once established, the patterns may create a personality which in itself is unstable with respect to interpersonal and/or intrapersonal relationships. The person's behaviour patterns might bring themselves into constant conflict with the people and environment around them.