The Minecraft character added to Super Smash Bros. Ultimate as DLC in October 2020 was Steve. Steve is the default playable avatar from Minecraft and joined the game as part of Fighters Pass Volume 2.
Steve’s inclusion brought Minecraft’s block-building and resource-gathering systems into Smash. His moveset uses materials mined from stages to craft weapons and tools, while blocks can be placed to create temporary platforms or barriers. His accompanying stage, Minecraft World, changes between several procedurally styled environments.
Zombie, Enderman, and Alex are not separate fighters. They are alternate appearances for the same Steve fighter slot, with Alex and the two undead-themed designs offering visual variety. This is why the question’s options can seem competitive even though they all point to Minecraft’s representation in the game.