<p>Strain modulation is a crucial way in engineering nanoscale materials. It is even more important for single photon emitters in layered materials, where strain can trap a delocalized exciton, leading to quantum emission. Herein, we apply strain by using the piezoelectric relaxor ferroelectric substrate. In addition to the strain-tuning of energy and polarization, we report on new observations, including the enhanced polarizability and tunable diamagnetic shift, from the charged localized excitons. As indicated from the polarization-resolved measurements, we attribute the formation of charged localized excitons to selenium vacancy defects. The shallow defect trap, supported by the value of <i>g</i>-factor, further allows for strain-modulation of the electron-hole overlap, hence resulting in the tunable diamagnetic shift. Our results provide a new perspective in integrating layered materials with functional substrates. The contrasting features observed from the charged localized excitons also signify the prospect of charged localized emitters for quantum science and technology.</p>