We assessed the extent to which feelings of sympathy and aggressive behaviors codeveloped from 6 to 12 years of age in a representative sample of Swiss children (N = 1,273). Caregivers and teachers reported children's sympathy and overt aggression in 3-year intervals. Second-order latent curve models indicated general mean-level declines in sympathy and overt aggression over time, although the decline in sympathy was relatively small. Importantly, both trajectories were characterized by significant interindividual variability. A bivariate second-order latent curve model revealed a small-moderate negative correlation between the latent slopes of sympathy and overt aggression, suggesting an inverse codevelopmental relationship between the constructs from middle childhood to early adolescence. In terms of predictive effects, an autoregressive cross-lagged model indicated a lack of bidirectional relations between sympathy and overt aggression, underscoring the primacy of the variables' rank-order stability. We discuss the codevelopment and developmental relations of sympathy and aggression, their potential conjoint social-emotional mechanisms, and the practical implications thereof.