Clemenza (Richard S. Castellano) is one of Don Corleone's main capos, or under-bosses, along with Tessio. He's a helpful mentor-figure to Michael, and a killer at the same time. He shows Michael the correct way to kill Sollozzo and McCluskey, and gives him advice both on his relationship with Kay and on cooking.
He also helps dispose of Vito Corleone's betrayer, Paulie, by having him whacked while Clemenza stops to urinate in New Jersey. He concludes the scene with the memorable line, "Leave the gun. Take the cannoli."