Walt Disney's Steamboat Willie features what iconic character?

Steamboat Willie is a 1928 American animated short film directed by Walt Disney and Ub Iwerks. It was produced in black and white by Walt Disney Studios and was released by Celebrity Productions. The cartoon is considered the debut of Mickey Mouse and his girlfriend Minnie, although both characters appeared several months earlier in a test screening of Plane Crazy. Steamboat Willie was the third of Mickey's films to be produced, but it was the first to be distributed, because Walt Disney, having seen The Jazz Singer, had committed himself to producing one of the first fully synchronized sound cartoons.

Source: Wikipedia