The structural and electronic properties of halide perovskites are strongly influenced by their defect chemistry, making precursor stoichiometry a simple route for controlling their material properties. Here, we investigate precursor-ratio engineered structural evolution in solution-processed, lead-free KCuCl_3-x thin films by systematically varying the KCl:CuCl₂ ratio. X-ray diffraction and Rietveld refinement reveal that the monoclinic KCuCl₃ framework is retained across the compositional series, while decreasing the KCl precursor concentration induces systematic anisotropic lattice expansion. Crystallographic occupancy analysis identifies preferential vacancy formation at the terminal chloride sites, establishing a direct relationship between precursor stoichiometry and site-selective defect formation. X-ray photoelectron spectroscopy and electron paramagnetic resonance further suggest progressive evolution of the Cu²⁺/Cu⁺ redox environment, demonstrating that chloride-ion vacancies (V_Cl) produce substantial electronic redistribution within the Cu-Cl framework. The accompanying narrowing of the optical band gap and increase in Urbach energy further indicate V_Cl-induced modification of the electronic structure and disorder.

The influence of this structural tuning extends to the surface electronic properties. Kelvin probe force microscopy shows a systematic evolution of the surface potential with increasing V_Cl concentration corresponding to a reduction in work function. Moreover, valence-band measurements indicate a progressive shift of the Fermi level toward the conduction band. These observations establish a direct connection between precursor-controlled lattice defects and the electronic energy landscape of KCuCl_3-x. The resulting changes in defect concentration and electronic structure also strongly influence device characteristics. Increasing V_Cl concentration promotes forming-free resistive switching, while an intermediate defect concentration provides an optimal balance between ionic transport and controlled conductance modulation, enabling highly linear synaptic potentiation and depression. Thus, precursor stoichiometry provides a versatile means of connecting crystal structure, defect chemistry, electronic properties, and functional behaviour in lead-free halide perovskites. Beyond optoelectronic tuning, the defect-controlled electronic landscape also opens pathways toward emerging device concepts such as resistive switching and neuromorphic functionality, underscoring the broader technological potential of this material system.