<p>Bullying represents a significant public health challenge during adolescence, with profound implications for victimized and perpetrating youth. Although prior research has extensively examined developmental pathways of bullying involvement, few studies have investigated how these pathways co-develop with prosocial bystander behaviors such as peer defending. Using three-wave longitudinal data from 1369 Chinese adolescents (50.8% female, <i>M</i>_age = 14.81, <i>SD</i> = 1.16), the current study employed parallel-process latent growth mixture modeling to identify joint developmental trajectories of bullying victimization/perpetration and defending behavior. Three distinct trajectory classes emerged in both the victimization-defending and perpetration-defending models: a high-defending group (14.83%–16.14%) characterized by low bullying involvement and high peer-nominated defending; a high-bullying involvement group (14.98%–21.62%) exhibiting elevated victimization or perpetration and low defending; and a normative group (63.55%–68.88%) with low bullying and moderate self-reported—but low peer-nominated—defending. Cross-classification revealed meaningful subgroups, including bully-victims and victim-defenders. Peer acceptance, mindful parenting, and social-emotional competency and acceptance goals increased adolescents’ likelihood of falling into the high-defending group, whereas peer rejection, harsh parenting, and popularity goals heightened their risk of being placed in the high-bullying involvement group. These findings advance a nuanced understanding of the heterogeneity in adolescent social roles and highlight the differential roles of ecological factors in shaping the co-development of bullying and defending behaviors during this critical developmental period.</p>