  Charge order is a central feature of the physics of cuprate superconductors
and is known to arise from a modulation of holes with primarily oxygen
character. Low-valence nickelate superconductors also host charge order, but
the electronic character of this symmetry breaking is unsettled. Here, using
resonant inelastic x-ray scattering at the Ni $L_2$-edge, we identify
intertwined involvements of Ni $3d_{x^2-y^2}$, $3d_{3z^2-r^2}$, and O
$2p_{\sigma}$ orbitals in the formation of diagonal charge order in an
overdoped low-valence nickelate La$_{4}$Ni$_{3}$O$_{8}$. The Ni $3d_{x^2-y^2}$
orbitals, strongly hybridized with planar O $2p_{\sigma}$, largely shape the
spatial charge distribution and lead to Ni site-centered charge order. The
$3d_{3z^2-r^2}$ orbitals play a small, but non-negligible role in the charge
order as they hybridize with the rare-earth $5d$ orbitals. Our results reveal
that the low-energy physics and ground-state character of these nickelates are
more complex than those in cuprates.
