Heavy metal contamination of aquatic systems poses persistent environmental and public health challenges. However, the mechanisms governing competitive adsorption of multiple metal ions on biomass-derived carbon materials remain insufficiently understood. In this study, fibrous activated carbon was synthesized from Acacia mangium leaf waste via one-step pyrolysis combined with KOH activation. The resulting material exhibited a high specific surface area (~800 m²/g) and a hierarchical micro–mesoporous structure. Competitive adsorption of Cu(II), Cd(II), and Ni(II) was systematically investigated in single- and multi-metal systems. Kinetic and isotherm models were evaluated using the corrected Akaike Information Criterion (AICc) to ensure statistically robust model selection. The Elovich model provided the lowest AICc values for all three metals, indicating heterogeneous surface chemisorption. In single-metal systems, adsorption affinity followed the order Cd(II) > Cu(II) > Ni(II), consistent with trends in hydration free energy. In contrast, under competitive conditions, selectivity shifted to Cu(II) > Cd(II) > Ni(II), suggesting that coordination stability plays a more dominant role in multicomponent systems. The results establish that adsorption selectivity on fibrous carbon is not intrinsic to individual ion properties but is dynamically redefined under competitive conditions by coordination stability effects.