Strongly correlated $4f$- and $5f$-electron systems often exhibit large effective masses due to cooperation between orbital hybridization and electron localization. In this context, the quadruple perovskite ruthenates $A{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ have attracted considerable interest as a rare example of heavy-fermion-like behavior arising from $d$ electron. However, the role of $A$-site cation substitution and its influence on the electronic structure remain unexplored. Here, we report a comprehensive study on the electronic properties of a newly synthesized ${\mathrm{CeCu}}_3{\mathrm{Ru}}_{3.25}{\mathrm{O}}_{12}$ sample, performing using synchrotron-based x-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). By optimizing the Ru content under high-pressure synthesis conditions, we obtained a metallic compound exhibiting the highest electronic specific heat coefficient ($\gamma =156\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$) within the $A{\mathrm{Cu}}_3{\mathrm{Ru}}_4{\mathrm{O}}_{12}$ family, together with a reduced effective magnetic moment (${\mu }_{\mathrm{eff}}=2.10{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$), indicating strong electronic correlations and substantial screening of local magnetic moments. The Ce $L_{\mathrm{III}}$-edge XAS measurements revealed an intermediate valence state (${\mathrm{Ce}}^{3.31+}$), indicative of $4f^0–4f^1$ hybridization. A chemical shift observed in the Cu $K$-edge absorption suggests a modified Cu valence state, likely arising from indirect $f–d$ interactions mediated by oxygen ligands. The PES measurements uncover a peak at $\left|\omega \right|\sim$ 19 meV near the Fermi level whose energy and intensity show pronounced temperature dependence below 100 K, resembling the evolution of Kondo resonance peaks seen in $f$-electron systems. These findings suggest the coexistence of $f–d$ and $d–d$ hybridization at different crystallographic sites is likely an important factor for the enhanced $\gamma$, reduced ${\mu }_{\mathrm{eff}}$, and Fermi-liquid behavior in ${\mathrm{CeCu}}_3{\mathrm{Ru}}_{3.25}{\mathrm{O}}_{12}$, offering a platform to explore correlated-electron phenomena in mixed-valent oxide systems.

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• Electronic structure
• Kondo effect
• Photoemission spectroscopy
• X-ray absorption spectroscopy