Personality are the enduring characteristics and behaviors that we use to adapt to life (this is the clinical psychology definition). Defense mechanisms are then a part of our personality. When defense mechanisms become malignant and maladaptive, this can become a personality disorder. In the Diagnostic and Statistical Manuel 5-TR, personality disorders are grouped into clusters that represent a common pattern of maladaptive thoughts and behaviors. Narcissism is in cluster B on the DSM.