The chapter explores the expression of problematic personality-based behaviors by individuals living with serious mental illness (SMI), emphasizing dimensional models of personality pathology. It defines personality as a dynamic configuration of traits shaped by biological, psychological, and sociocultural factors, and outlines the Five-Factor Model (FFM) as a foundational framework. Maladaptive traits, when rigid, pervasive, and distressing, are linked to functional impairment and are increasingly conceptualized dimensionally through models such as the DSM-5-TR Alternative Model for Personality Disorders (AMPD) and ICD-11. These models assess severity and trait domains including negative affectivity, detachment, antagonism, disinhibition, and psychoticism or anankastia. Epidemiological data suggest early onset and lifelong impact, with increased risk for co-occurring mental health problems and under-recognition in clinical settings. Behavioral strategies emphasize adaptive trait expression and contextual responsiveness. Dimensional, trait-based approaches to identification and developing individualized support strategies to align personality science with clinical utility and global relevance.