  It is shown that the spectral amplitude of gravitational-wave (GW) ringdown
of a Kerr black hole sourced by an extreme mass ratio merger can be modeled by
the $\textit{greybody factor}$, which quantifies the scattering nature of the
black hole geometry. The estimation of the mass and spin of the remnant is
demonstrated by fitting the greybody factor to GW data without using black hole
quasi-normal modes. We propose that the ringdown modeling with the greybody
factor may strengthen the test of gravity as one can avoid the possible
overfitting issue and the start time problem in the ringdown modeling with
superposed quasi-normal modes.
