  Interactions between plasma particles and electromagnetic waves play a
crucial role in the dynamics and regulation of the state of space environments.
From plasma physics theory, the characteristics of the waves and their
interactions with the plasma strongly depend on the composition of the plasma,
among other factors. In the case of the Earth's magnetosphere, the plasma is
usually composed by electrons, protons, O+, and He+ ions, all with their
particular properties and characteristics. Here, using plasma parameters
relevant for the inner magnetosphere we study the dispersion properties of
Kinetic Alfv\'en Waves (KAW) in a plasma composed by electrons, protons, He+
ions, and O+ ions. We show that heavy ions induce significant changes to the
dispersion properties of KAW, such as polarization, compressibility, and
electric-to-magnetic amplitude ratio, and therefore the propagation of Kinetic
Alfv\'en Waves is highly determined by the relative abundance of He+ and O+ in
the plasma. These results when discussed in the context of observations in the
Earth's magnetosphere, suggest that for many types of studies based on theory
and numerical simulations, the inclusion of heavy ions should be customary for
realistic modeling of plasma phenomena in the inner magnetosphere or other
space environments in which heavy ions can contribute with a substantial
portion of the plasma such as planetary magnetospheres and comet plasma tails.
