This article presents a detailed case study examining the psychological profile and treatment of “James,” a client with co-occurring substance use disorder (SUD) and posttraumatic stress disorder (PTSD), through the lens of the Five-Factor Model (FFM) of personality. By integrating multimodal assessment data—including the SASSI-4, TSI-2, MMPI-2, and MCMI-IV—with FFM trait analysis, the case illustrates how personality factors such as high Neuroticism, low Agreeableness, and low–average Conscientiousness contribute to treatment complexity and inform targeted interventions. The article also explores the ethical and clinical implications of applying a harm-reduction approach during early recovery, offering a rare, real-world example of its implementation. Through a critical discussion of therapist decision-making, client autonomy, and treatment risks, the article provides a model for ethically navigating unconventional strategies in dual-diagnosis cases. This case contributes to emerging paradigms in trauma-informed, personality-integrated addiction treatment and invites further exploration into precision-based approaches in mental health care.