We examined family SES inequalities in non-cognitive skills among young Korean adolescents. Using nationally representative data for 6th graders and 9th graders between 2004 and 2014 and school-fixed effects models, we showed how internal motivation, self-esteem, sociability, perseverance, and the composite score from those four indicators differ across students with varying family SES. Additionally, we examined how family SES gaps vary by grade and test score, as well as across the quantiles of non-cognitive skills. Our analyses uncover several significant new pieces of evidence regarding educational inequality. To begin, we find robust family SES inequalities across all non-cognitive outcomes regardless of model or sample specification. However, we find no discernible systematic trends in the decade following 2004. In terms of the role of academic performance, we find that academic performance partially but considerably explains family SES differences in non-cognitive outcomes. More than half of total family SES gaps persist among students with identical test scores. Additionally, we find evidence that academic performance significantly moderates family SES inequality in non-cognitive skills. Family SES gaps are greatest at the lowest scores and at the highest scores, implying that both compensatory and reinforcing family advantages are at play. For 9th graders, we find strong evidence that the correlation between non-cognitive skills and academic performance is unequally distributed across family SES groups and increases over time. Finally, we find that family SES disparities are greatest at the top of the non-cognitive score distribution, which is largely explained by the advantage of the top quartile SES families.