$\mathrm{CuCl}$ has more covalent character than $\mathrm{NaCl}$ because

$\mathrm{CuCl}$ has more covalent character than $\mathrm{NaCl}$ because ${\mathrm{Cu}}^{+}$ has more polarizing power than ${\mathrm{Na}}^{+}$.

The covalent character is high for a compound if the polarizing power of the cation is higher.