We examined developmental trends and sources of stability and change in adolescent personality by using twin data collected from 1981 to 2010 (273 monozygotic (MZ) and 48 dizygotic (DZ) twin pairs) from a secondary school affiliated with the University of Tokyo. Phenotypic analyses showed high rank-order stability and substantial mean-level increases in neuroticism and declines in extraversion over the adolescent years. Longitudinal bivariate genetic analyses revealed that the best-fitting model for adolescent personality includes additive genetic and non-shared environmental influences. Heritability estimates ranged approximately from 0.30 to 0.60. Additionally, three-year stability in adolescent personality was influenced mainly by genetic factors, and there were both genetic and environmental innovations in mid-adolescence. Our findings suggest that both genetic and environmental effects have significant roles in the etiology of personality development across adolescence.