  A kilonova is a short-lived explosive event in the universe, resulting from
the merger of two compact objects. Despite its importance as a primary source
of heavy elements through r-process nucleosynthesis, its nature is not well
understood, due to its rarity. In this work, we introduce a model that
determines the density of a radially-stratified relativistic ejecta. We apply
the model to kilonova ejecta and explore several hypothesized velocity profiles
as a function of the merger's ejection time. These velocity profiles result in
diverse density profiles of the ejecta, for which we conduct radiative transfer
simulations using TARDIS with the solar r-process composition. Consequently, we
investigate the impact of the ejecta velocity profile on the resulting
lightcurve and spectral evolution through the line transitions of heavy
elements. The change in the rate at which these elements accumulate in the
line-forming region leaves its imprint on the kilonova lightcurve at specific
wavelengths, causing the lightcurves to decay at different rates. Furthermore,
in several profiles, plateau-like behaviors (slow and/or flat decline) are also
observed. In conclusion, this work proposes potential scenarios of the kilonova
evolution due to the ejecta velocity profile.
