  The $g$-factor anisotropy of the heavy quasiparticles in the hidden order
state of URu$_2$Si$_2$ has been determined from the superconducting upper
critical field and microscopically from Shubnikov-de Haas (SdH) oscillations.
We present a detailed analysis of the $g$-factor for the $\alpha$, $\beta$ and
$\gamma$ Fermi-surface pockets. Our results suggest a strong $g$-factor
anisotropy between the $c$ axis and the basal plane for all observed Fermi
surface pockets. The observed anisotropy of the $g$-factor from the quantum
oscillations is in good agreement with the anisotropy of the superconducting
upper critical field at low temperatures, which is strongly limited by the
paramagnetic pair breaking along the easy magnetization axis $c$. However, the
anisotropy of the initial slope of the upper critical field near $T_c$ cannot
be explained by the anisotropy of the effective masses and Fermi velocities
derived from quantum oscillations.
