  In this work we investigate the quantitative effects of the misalignment
kinetic axion on $R^2$ inflation. Due to the fact that the kinetic axion
possesses a large kinetic energy which dominates its potential energy, during
inflation its energy density redshifts as stiff matter fluid and evolves in a
constant-roll way, making the second slow-roll index to be non-trivial. At the
equations of motion level, the $R^2$ term dominates the evolution, thus the
next possible effect of the axion could be found at the cosmological
perturbations level, via the second slow-roll index which is non-trivial. As we
show, the latter elegantly cancels from the observational indices, however, the
kinetic axion extends the duration of the inflationary era to an extent that it
may cause a 15$\%$ decrease in the tensor-to-scalar ratio of the vacuum $R^2$
model. This occurs because as the $R^2$ model approaches its unstable quasi-de
Sitter attractor in the phase space of $F(R)$ gravity due to the $\langle R^2
\rangle $ fluctuations, the kinetic axion dominates over the $R^2$ inflation
and in effect the background equation of state is described by a stiff era, or
equivalently a kination era, different from the ordinary radiation domination
era. This in turn affects the duration of the inflationary era, increasing the
$e$-foldings number up to $5$ $e$-foldings in some cases, depending on the
reheating temperature, which in turn has a significant quantitative effect on
the observational indices of inflation and especially on the tensor-to-scalar
ratio.
