Background/Objectives: Body mass index (BMI) cannot distinguish fat mass (FM) from fat-free mass (FFM). The fat-free mass index (FFMI = FFM/height²) and fat mass index (FMI = FM/height²) decompose BMI into lean and fat components. We aimed to establish the first nationally representative, BIA-based FFMI and FMI reference values for Korean adults, visualize body composition trajectories, and classify obesity subtypes using the body composition chart. Methods: Cross-sectional data from 10,140 participants (4508 men, 5632 women; aged 10–80 years) from KNHANES IX (2022–2023) were analyzed. Multifrequency BIA (InBody 970) measured FFM and FM. Obesity subtypes (%BF ≥ 25% men, ≥35% women) were classified as underlean (FFMI < P5), proportional (P5–P95), or heavy (>P95) relative to an 18–59-year-old reference. Results: In men, FFMI peaked at 19.1 kg/m² (30–49 years), declining 9.7% by 70–80 years, while FMI remained stable. In women, FMI increased 44% with stable FFMI. Underlean obesity in men rose from 0.7% (30–39 years) to 17.4% (70–80 years), undetected by BMI or waist circumference. Conclusions: These reference values and body composition chart provide practical tools for identifying underlean obesity and assessing body composition beyond BMI in Korean adults.