  We present a high-resolution in-beam $\gamma$-ray spectroscopy study of
excited states in the mirror nuclei $^{55}$Co and $^{55}$Ni following
one-nucleon knockout from a projectile beam of $^{56}$Ni. The newly determined
partial cross sections and the $\gamma$-decay properties of excited states
provide a test of state-of-the-art nuclear structure models and probe mirror
symmetry in unique ways. A mirror asymmetry for the partial cross sections
leading to the two lowest $3/2^-$ states in the $A = 55$ mirror pair was
identified as well as a significant difference in the $E1$ decays from the
$1/2^+_1$ state to the same two $3/2^-$ states. The mirror asymmetry in the
partial cross sections cannot be reconciled with the present shell-model
picture or small mixing introduced in a two-state model. The observed mirror
asymmetry in the $E1$ decay pattern, however, points at stronger mixing between
the two lowest $3/2^-$ states in $^{55}$Co than in its mirror $^{55}$Ni.
