Personality, there are many different ones out there for the number of people that there are, and it can be like undoing a combination lock without having the code when it comes to understanding others.
For like any combination locks, some are easier to open:
Fingerprint Safe Jiggled Open – Lock Picking Lawyer
Others are much harder, an enigma so to speak:
Vending Machine Puzzle – Chris Ramsey
Though there are certain parameters, categories or archetypes that people fall under and group into to that go into creating an individuals personality.
Personalities are as individual as the people that are involved, though like finger prints, there are patterns of swirls and curls, that when one can read them, the real person begins to show