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Doomguy, also known as the Doom Marine (or the Doom Slayer in later Doom games), is the protagonist of the Doom video game franchise.
In SMG4: If Mario Was In Animal Crossing, after Mario destroys Tom Nook's deserted island, Isabelle calls Doomguy to save them. As Mario was running from Doomguy, Mr. Resetti throws a pickaxe at him. Doomguy and Isabelle then catches up to Mario and kicks Mario somewhere far away.
In SMG4: Stupid Mario Arcade, when Mario was stuck in the Doom arcade where he saw him shooting at monsters, Mario pushed him to get to kill the princess leaving him to be attacked by monsters he was shooting at.