We report ${}^{15}\mathrm{N}$ nuclear magnetic resonance (NMR) and the nitrogen-isotope effect studies on c-axis oriented sample of Li-doped HfNCl with superconducting transition temperature ${T}_{c}\ensuremath{\sim}25.5\mathrm{K}.$ ${}^{15}\mathrm{N}$ NMR Knight shift decreases toward zero below ${T}_{c},$ giving evidence that the pairing symmetry is an even-parity spin singlet, and that the nitrogen site plays an important role in the occurrence of superconductivity. A nitrogen isotope shift is found to be quite small, $\ensuremath{\Delta}{T}_{c}\ensuremath{\sim}0.1\mathrm{K}$ $({\ensuremath{\alpha}}_{N}=0.07\ifmmode\pm\else\textpm\fi{}0.02),$ which cannot reproduce such a high ${T}_{c}$ within the traditional BCS framework.