The decrease of spin polarization in spintronics devices under the application of a bias voltage is one of a number of currently important problems that should be solved. Here, an unprecedented robustness of the spin polarization in multilayer-graphene spin valves at room temperature is revealed. Surprisingly, the spin polarization of injected spins is constant up to a bias voltage of +23 V and -0.6 V in positive- and negative-bias voltage applications at room temperature, respectively, which is superior to all spintronics devices. This finding is induced by suppression of spin scattering due to an ideal-interface formation. Furthermore, an importatant accordance between theory and experiment in molecular spintronics is found by observing the fact that the signal intensity in a local scheme is double that in a nonlocal scheme, as theory predicts, which provides construction of a steadfast physical basis in this field.