Despite exhibiting a similar $d_{x^2-y^2}$ band character to cuprates, infinite-layer nickelates host additional electron pockets that distinguish them from single-band cuprates. The elusive orbital origin of these electron pockets has led to competing theoretical scenarios. Here, using polarization-dependent and resonant angle-resolved photoemission spectroscopy, we determine the orbital character of the Fermi surfaces in ${\mathrm{NdNiO}}_2$. Our data reveal that the electronlike pocket arises predominantly from interstitial $s$ states, with negligible contributions from rare-earth $5d$ and $4f$ orbitals near the Fermi level. The observation of well-defined quantum-well states indicates a uniform distribution of these interstitial electrons throughout the film thickness. By comparing with electronic structure of ${\mathrm{LaNiO}}_2$, we find that the rare-earth element modulates the Ni-derived bands and hopping integrals through a chemical pressure effect. These findings clarify the role of rare-earth elements in shaping the low-energy electronic structure and establish the presence of electridelike interstitial $s$ states in a correlated oxide system, where electrons occupy lattice voids rather than atomic orbitals. The electridelike character offers new insight into the self-doping and superconductivity in infinite-layer nickelates.

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• Electronic structure
• Fermi surface
• High-temperature superconductors
• Nickelates
• Thin films
• Angle-resolved photoemission spectroscopy