<p>Adolescence is a critical period for the emergence of personality disorder traits. However, differentiating emerging symptoms of borderline personality from general adolescent turmoil is a significant challenge and developmental trajectories of borderline personality in adolescence, especially among boys, remain poorly understood. This study examined links between emotion regulation difficulties and aggression, two key features of borderline personality disorder (BPD) in adults, and borderline personality features. Further, it assessed whether anger rumination uniquely related to BPD symptoms. Among 101 adolescent boys participating in a military-style residential program for at-risk youth (65.3% White/Caucasian, <i>M</i> age = 16.68), results identified strong links between emotion regulation difficulties, aggression, and borderline personality features. Anger rumination was also a significant factor over and above emotion regulation difficulties and aggression. Further, specific dimensions of emotion regulation difficulties including limited access to emotion regulation strategies, difficulties with goal-directed behavior, and lack of emotional clarity were identified as particularly relevant to BPD traits among these adolescent boys. These findings underscore the importance of early identification of BPD traits in adolescence, as these steps may mitigate serious long-term outcomes and broader public health costs. These results have implications for early screening and targeted intervention of emerging BPD traits in adolescent boys. Anger-specific processes may be particularly relevant to BPD development among boys, suggesting a tangible target for early intervention.</p>