Research has consistently found high rates of comorbidity between narcissistic personality disorder (NPD) and borderline personality disorder (BPD). Patients with this complex clinical presentation often present formidable challenges for clinicians, such as intense devaluation, entitlement, and exploitation. However, there is a significant gap in the literature in identifying the clinical characteristics of these NPD/BPD patients. In this article, we present recent research describing patients with comorbid NPD/BPD, as compared with patients with BPD without NPD (BPD), from two randomized clinical trials for the treatment of borderline personality disorder, with a particular emphasis on attachment status and mentalization. We anchor our discussion of these patients in object relations and attachment theory, and we describe our treatment approach, transference focused psychotherapy (TFP). We conclude by using case material to illustrate our research findings, highlighting the significant differences between patients with NPD/BPD and BPD/non-NPD in terms of their attachment classification.