Minecraft wiki entry for Super_Smash_Bros._Ultimate, associated text: Steve is referred within the game files with the codename "pickel". In Japanese, the loanword pikkeru is used to refer to ice axes, deriving from the German word "Eispickel".
According to former Minecraft production director Daniel Kaplan, it took at least 5 years of negotiating between Nintendo and Mojang Studios for Steve to appear in Smash.[2]
Steve's alternate costumes each consist of a single texture taken from Minecraft and modified slightly presumably to account for the different lighting in Super Smash Bros. Ultimate. This notably makes modifying them through file replacement extremely simple.