Leonard (Martin Landau)’s Timeline and Summary

Leonard helps Vandamm at the Townsend estate, where they first interrogate Thornhill, then force him to drive drunk.
He travels with Vandamm to Chicago and is by his side after that as well, always watching out for him.
Ever protective of his boss, Leonard tells Vandamm that Eve's deceiving him and working with whoever's working with Thornhill. Leonard shows Vandamm the gun that Eve has used to shoot Thornhill, which shoots only blanks.
Leonard inadvertently points Thornhill to the statuette that's hiding the information that Vandamm's going to smuggle out of the country.
Leonard chases Thornhill and Eve across Mount Rushmore and very nearly succeeds in killing them.
He's shot just in time, though, and falls to his death.