This study investigated the co-development of the ideal L2 writing self, writing enjoyment, and writing anxiety over time, as well as their predictive role in L2 writing achievement, using latent growth curve modeling (LGCM). A total of 145 EFL students in an IELTS writing course completed scales for these constructs across four one-month intervals. Polynomial LGCMs were applied to compare linear, piecewise, and quadratic growth models. Results revealed linear growth with increasing trends from T1 to T4 for the ideal L2 writing self and writing enjoyment, while writing anxiety followed a piecewise trajectory, with an initial decline (T1-T2) followed by stabilization (T2-T4). Significant covariances among the three constructs indicated their interconnected development over time. Initial levels of these factors were not predictive of L2 writing achievement, but their growth trajectories significantly predicted outcomes. These findings highlight the dynamic interplay of emotional and motivational factors in L2 writing development and underscore the importance of longitudinal research in capturing their evolving influence on L2 writing achievement.