Short answers, pulled from the story.
A personality disorder is a mental disorder characterized by an enduring and pervasive maladaptive pattern of behavior, emotions, cognition, and inner experience. This definition appears in the sixth chapter of the International Classification of Diseases and the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders.
These patterns develop early in life and are inflexible. The behavior patterns are typically recognized by adolescence, the beginning of adulthood, or sometimes even childhood.
The categorical approach views personality disorders as discrete entities that are distinct from each other as well as from normal personality. In contrast, the dimensional approach suggests that personality disorders exist on a continuum, with traits varying in degree rather than kind.
Environmental factors play a significant role in development and include prenatal conditions, childhood trauma, abuse, neglect, and other adverse childhood experiences. A study of 793 mothers and children found that children who experienced verbal abuse were three times as likely to have borderline, narcissistic, obsessive-compulsive, or paranoid personality disorders in adulthood.
Individual psychotherapy has been a mainstay of treatment and is evidence-based for various types such as avoidant, obsessive-compulsive, and dependent personality disorders. Dialectical behavior therapy evolved into a process where the therapist and client work with acceptance and change-oriented strategies and was recommended by the NICE review for BPD symptoms.