To evaluate the importance of the electronic structure of CuA to its electron-transfer (ET) function, a quantitative description of the ground-state wave function of the mixed-valence (MV) binuclear CuA center engineered into Pseudomonas aeruginosa azurin has been developed, using a combination of S K-edge and Cu L-edge X-ray absorption spectroscopies (XAS). Parallel descriptions have been developed for a binuclear thiolate-bridged MV reference model complex ([(LiPrdacoSCu)2]+) and a homovalent (II,II) analogue ([LiPr2tacnSCu)2]2+, where LiPrdacoS and LiPr2tacnS are macrocyclic ligands with attached thiolates that bridge the Cu ions. Previous studies have qualitatively defined the ground-state wave function of CuA in terms of ligand field effects on the orbital orientation and the presence of a metal-metal bond. The studies presented here provide further evidence for a direct Cu-Cu interaction and, importantly, experimentally quantify the covalency of the ground-state wave function. The experimental results are further supported by DFT calculations. The nature of the ground-state wave function of CuA is compared to that of the well-defined blue copper site in plastocyanin, and the importance of this wave function to the lower reorganization energy and ET function of CuA is discussed. This wave function incorporates anisotropic covalency into the intra- and intermolecular ET pathways in cytochrome c oxidase. Thus, the high covalency of the Cys-Cu bond allows a path through this ligand to become competitive with a shorter His path in the intramolecular ET from CuA to heme a and is particularly important for activating the intermolecular ET path from heme c to CuA.