Background: Despite extensive research examining features of adolescents' ego-identity development, little research has demonstrated patterns of ego-identity development and its relationship with various individual and environmental factors that are theoretically important for identity development. Objective: The current study investigated developmental trajectories of ego-identity and the longitudinal relationship between multiple predictors (Korean language proficiency, sex/gender, logged household income, and child maltreatment) and ego-identity development. Methods: Data from three time points (Time 1 = 9th grade; Time 2 = 11th grade; Time 3 = 12th grade) of the Korean Children and Youth Panel Survey (N = 7053) were used. The mean age of participants in each wave was 15, 17, and 18 years old. Results: The results showed that the level of ego-identity development varied across times--that the developmental pattern plateaued and decreased slightly after Time 2. A significant association between Korean language proficiency and ego-identity development was found, indicating that individuals who reported a higher level of Korean language proficiency displayed a greater level of ego-identity over the three observation periods. Sex/gender, logged household income, and child maltreatment were significantly associated with ego-identity development at the initial level. Conclusion: The findings highlight the importance of identifying patterns of ego-identity development and understanding its association with multiple predictors.