  Thanks to the high quality data of space missions, the detection of mixed
modes has become possible in numerous stars. In this work, we investigate how
the mixing character of dipolar mixed modes changes with stellar evolution, as
well as with frequency within each stellar model. This is achieved by
monitoring the variations in the coupling strength and the period spacing of
dipolar mixed modes in red-giant models. These parameters are measured by
fitting the asymptotic expansion of mixed modes to the model frequencies of a
grid of red-giant models with masses between 1.0 and 2.0 $M_\odot$ and three
different chemical abundances. The coupling strength and the period spacing
decrease with stellar evolution. We find that the slopes of their decreasing
trends depend on the radial order of the pressure mode component. A
non-negligible increase of the coupling strength with frequency by up to around
40\% is found in the observable frequency range for a set of red-giant models.
On the contrary, no significant changes of the period spacing with frequency
are found. The changes in the mixing character of the modes are in most cases
affected by the model mass and metallicity. Buoyancy glitches also have an
impact on the mixing character. Significant fluctuations in the estimated
coupling strength and period spacing are found for models approaching the
luminosity bump, if the glitch impact of the frequencies is not considered in
the applied asymptotic expansion.
