We present the analysis of the stellar populations and kinematics of the globular cluster (GC)-rich ultra-diffuse galaxy (UDG), PUDG-R21, using spectroscopic observations obtained with the Keck Cosmic Web Imager. The recessional velocity is measured to be 5536 $\pm$ 10 km s‑1, confirming its association with the Perseus Cluster. The galaxy exhibits mild rotation of 15.6 $\pm$ 10 km s‑1 and a stellar velocity dispersion of 19.4 $\pm$ 3.5 km s‑1 within the galaxy effective radius. From this, we infer a dynamical mass of $M_{\mathrm{dyn}}=9.3\pm 3.3\times 10^{8} \, {\rm M}_{\odot }$. Based on a halo mass derived from PUDG-R21 GC counts, we find our dynamical mass is consistent with a cored dark matter profile. The integrated stellar population analysis reveals a predominantly old stellar population of $10.4\pm 1.2$ Gyr, with intermediate–low metallicity ($[{\rm M/H}] = -0.64\pm 0.12$ dex) and elevated $\alpha$ abundances ($[{\rm Mg/Fe}] = 0.38\pm 0.25$ dex). The inferred star formation history suggests rapid stellar assembly, likely truncating prior to or during the galaxy's infall into the cluster at an early epoch (${\sim} 10$ Gyr ago). The analysis of stellar population gradients (age and metallicity) indicates a flat profile out to one effective radius. Here, we consider the involvement of two star formation events, initially forming a large population of metal-poor GCs, and then the latter contributing to the more metal-enriched diffuse stellar body. The evidence of subsequent star formation suggests this galaxy is more like an extension of the classical dwarf population than the much-discussed failed galaxy UDGs.