  Kagome superconductors AV$_3$Sb$_5$ (A = K, Rb, Cs) provide a fertile
playground for studying intriguing phenomena, including non-trivial band
topology, superconductivity, giant anomalous Hall effect and charge density
wave (CDW). Recently, a $C_2$ symmetric nematic phase prior to the
superconducting state in AV$_3$Sb$_5$ drew enormous attention due to its
potential inheritance of the symmetry of the unusual superconductivity.
However, direct evidence on the rotation symmetry breaking of the electronic
structure in the CDW state from the reciprocal space is still rare, and the
underlying mechanism remains ambiguous. The observation shows unconventional
unidirectionality, indicative of rotation symmetry breaking from six-fold to
two-fold. The interlayer coupling between adjacent planes with $\pi$-phase
offset in the 2$\times$2$\times$2 CDW phase leads to the preferred two-fold
symmetric electronic structure. These rarely observed unidirectional
back-folded bands in KV$_3$Sb$_5$ may provide important insights into its
peculiar charge order and superconductivity.
