The proton is the smallest, lightest, hardest and most versatile Lewis acid.
However, the proton is never observed free (in chemistry at least, high energy high vacuum physics is different). The proton is always passed or transferred from one Lewis base to another in a concerted Brønsted acid/base proton transfer reaction.
The proton has so little mass that it (partially) quantum tunnels between complexed states, and the ability of a species to complex with a proton defines Lewis base character.
Brønsted acids are all proton/Lewis bases complexes: the proton is the agent of Brønsted acidity.
The Ka and pKa of are a measure of Brønsted acid strength with respect to water. As the Lewis acid H+ remains constant, the terms Ka and pKa are a measure of a conjugate (Lewis) base's affinity for H+ with respect to the standard Lewis base water, :OH2.