Bystanders shape bullying dynamics, yet little is known about how their behaviors change over time and related socio-moral factors. This study adopted a person-centered longitudinal approach to examine joint trajectories of bystander behaviors and their co-evolution with moral disengagement and conformity to peer pressure. A sample of 994 Spanish adolescents (48% girls; Mage T1 = 12.5, SD = 0.6) was surveyed across three waves. Parallel-process Latent Class Growth Analysis identified three trajectories: Sustained Defending (defending declined slightly at the end of secondary education), Increase Pro-bullying (pro-bullying gradually increased), and Decrease Pro-bullying (pro-bullying decreased from the first wave). These behavioral shifts were linked to specific socio-moral mechanisms: in the Increase Pro-bullying profile, persistently high moral disengagement and rising conformity co-occurred with the transition from passivity to pro-bullying, while in the Decrease Pro-bullying profile, reductions in both processes co-occurred with the shift toward defending. Findings provide evidence of the variability in behaviors adopted by bystanders across adolescence, with a significant change in pro-bullying patterns from 8^th to 9^th Grade. The identification of bystanders’ trajectories and the coevolution of sociomoral processes support the reconceptualization of bystanding as a dynamic developmental phenomenon shaped by intrapersonal and interpersonal forces and highlighting targets for long-term intervention.