  Destructive interference between electron wavefunctions on the
two-dimensional (2D) kagome lattice induces an electronic flat band, which
could host a variety of interesting many-body quantum states. Key to realize
these proposals is to demonstrate the real space localization of kagome flat
band electrons. In particular, the extent to which the often more complex
lattice structure and orbital composition of realistic materials counteract the
localizing effect of destructive interference, described by the 2D kagome
lattice model, is hitherto unknown. We used scanning tunneling microscopy (STM)
to visualize the non-trivial Wannier states of a kagome flat band at the
surface of CoSn, a kagome metal. We find that the local density of states
associated with the flat bands of CoSn is localized at the center of the kagome
lattice, consistent with theoretical expectations for their corresponding
Wannier states. Our results show that these states exhibit an extremely small
localization length of two to three angstroms concomitant with a strongly
renormalized quasiparticle velocity, which is comparable to that of moir\'e
superlattices. Hence, interaction effects in the flat bands of CoSn could be
much more significant than previously thought. Our findings provide fundamental
insight into the electronic properties of kagome metals and are a key step for
future research on emergent many-body states in transition metal based kagome
materials.
