Defects in two-dimensional (2D) semiconductors play a decisive role in determining their electronic, optical, catalytic and quantum properties. Understanding how defect energy levels respond to variations in layer thickness is essential for achieving reproducible and scalable device performance. We report the persistence of layer-tolerant defect levels in rhenium disulfide $\left({\mathrm{ReS}}_2\right)$, where both donor- and acceptor-type charge transition levels remain nearly unchanged from monolayer to bulk in both AA and AB stacking. The associated two-level quantum system also retains its character across thicknesses, enabling ${\mathrm{ReS}}_2$ to serve as a platform for layer-tolerant single-photon emitters. The invariance arises from the interplay between electronic energy minimization and structural relaxation, which together counteract quantum confinement and reduced dielectric screening. Additionally, the intrinsically weak interlayer coupling in ${\mathrm{ReS}}_2$ plays a crucial role. Our findings uncover the microscopic origin of this unique behavior, distinguishing ${\mathrm{ReS}}_2$ from other transition-metal dichalcogenides and highlighting its potential for thickness-independent optoelectronic and quantum photonic applications.

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• Defects
• Dopants
• Vacancies
• 2-dimensional systems
• Transition metal dichalcogenides
• Density functional theory
• First-principles calculations