Background: Labeling theory underscores the sequential process that unfolds within an individual, influenced by those in his or her immediate surroundings. Nevertheless, research into the developmental characteristics of labeling theory remains limited. Furthermore, prior studies investigating the impact of informal labeling have predominantly occurred within a Western cultural context. Objective: This study explores two fundamental aspects of within-individual dynamics in the influence of perceived informal labeling on deviant self-concept, utilizing a sample of Korean youth: (1) the evolution of a labeled youth's deviant self-concept over time and (2) variations in the development of the deviant self-concept among individuals. Methods: Using HLM 8.0, A series of random-effects models were estimated using six-wave panel data from Korean youth. Results: The findings reveal that (1) the immediate within-individual effect of perceived informal labeling on deviant self-concept was significantly positive. However, when one- and two-year time intervals between variables were considered, the effects showed a reverse significance. (2) The increase in deviant self-concept over a six-year span was less pronounced for those with a higher level of perceived informal labeling in the first wave, suggesting that these youths attempted to mitigate the adverse effects of labeling. Conclusions: The impact of perceived informal labeling is not consistent over time. Moreover, labeling effects may be shaped by an individual's cultural background. This study is significant as it offers empirical testing of the developmental aspects of labeling theory within a unique cultural context.