Developing permanent magnet alloys with decreased critical elements (e.g., Nd, Dy, and Co) requires identifying compositions and structures with uniaxial magneto-crystalline anisotropy energy (MAE), large magnetization, and a high ferromagnetic transition temperature (Curie temperature - TC). One approach to minimizing the critical elements in potential permanent magnet alloys is to use overly produced Ce, which is less critical. Furthermore, reducing Co content in RCo5 (R = Rare Earth) alloys is necessary since Co is also a critical element. An obvious choice for decreasing Co content is a substitution with non-critical Fe. However, the Fe is not stable in the lattice due to the reduced number of d-electrons. Concomitant substitution of Cu stabilizes Fe substitution. Employing first-principles electronic structure theory, we identify the weakly localized nature of cobalt in CeCo5, which causes high uniaxial magnetic anisotropy of ∼10 MJ/m3. In contrast, substituted Cu delocalizes the Co’s 3d-states, resulting in lower anisotropy. Calculations show that 10% Cu can stabilize 20% Fe substituted for Co, which significantly enhances magnetic moment in the Ce (Co, Fe, Cu)5. This prediction is in good agreement with a single-crystal experiment in which the optimal composition was identified to be 15% of Fe and 12% Cu. The competitive non-equivalent Co sites preferred by Cu and Fe, a unique electronic structure including exchange and crystal field splitting, and rigid band shift variation are borne by 3d states of Co, Fe, and Cu around the Fermi level, all play an essential role in tuning the magnetic properties.