Flat bands near the Fermi energy ($E_{\mathrm{F}}$) enhance the density of states and promote strong electronic correlations, often driving novel quantum phases. Understanding the microscopic origin of these correlations is essential for uncovering the mechanisms behind correlated phenomena such as superconductivity and magnetism. Here, we investigate the electronic structure of the three-dimensional pyrochlore superconductor ${\mathrm{CeRu}}_2$ using angle-resolved photoemission spectroscopy. The Ru sublattice forms a three-dimensional kagome network, theoretically predicted to host flat bands. While we resolve dispersive Ru $4d$–derived bands near $E_{\mathrm{F}}$, their high band velocities suggest a minimal contribution to low-energy correlations. In contrast, we observed clear signatures of strong hybridization between Ce $4f$ and Ru $4d$ states at $E_{\mathrm{F}}$. Our results identify Ce $4f$ electrons as the primary source of strong correlations in ${\mathrm{CeRu}}_2$, providing crucial insight into the microscopic origin of its superconductivity and magnetic phenomena.

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• Flat bands
• Superconductivity
• Heavy-fermion systems
• Kagome metal
• Pyrochlores
• Strongly correlated systems
• Angle-resolved photoemission spectroscopy