Doomguy, also known as the Doom Marine (or the Doom Slayer in later Doom games), is the protagonist of the Doom video game franchise.
Prior to Shroomy's alteration, Doomguy was a loving and cherishing family member to him since childhood and would entertain Shroomy in anyway possible such as sending demons for him to kill or reading a bedtime story. He fell into a state of depression after Shroomy was permanently changed.
However as the latest episode, Doomguy has disowned Shroomy and developed a preference to Isabelle instead, showing he has become selfish and unfaithful towards him along with the rest of his family. He even tells Shroomy that he only liked who he was in the past and hated the Shroomy he became after "the bite of '87". Even after his nephew calls him out for being a neglectful husband, father and uncle, Doomguy outright ignores him and continues to fondle over Isabelle.
As revealed in SMG4: Mario Babies, Doomguy was Shroomy's uncle. He loved Shroomy for his thirst for demon hunting, but on Shroomy's birthday, in 1987, when Doomguy took Shroomy to Freddy Fazbear's Pizza and activated the animaltronics' evil modes to have a demon hunting fiesta, Shroomy got bitten by Golden Freddy. The bite left a huge hole in Shroomy's head, giving him brain damage and causing his personality to change from being a bloodthirsty demon killer to a cheery boy scout, much to Doomguy's horror.
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.
SMG4: Mario and the VILLAGE reveals that he ditched Shroomy altogether for not being who he used to be and replaced him with Isabelle. His obsession over Isabelle went to the point where she's all he cares about, causing Alcina Dimitrescu and her daughters to hate the cute dog. He doesn't even care when Shroomy gave up trying to get his uncle to accept him.
In SMG4: She's Back!. it is revealed that his wife divorced him after he accidentally clicked "Cast to Family Room TV" while watching porn on the Internet.
- Him being a contact with Isabelle is an allusion to the fan art trend where those two are put together despite their universe being opposite theme. This is possibly due to Doom Eternal and Animal Crossing: New Horizons being released on the same day: March 20th, 2020.
- It is revealed in SMG4: War On Beeg SMG4 that Shroomy worked alongside him to fight demons, but left to become a boy scout after Shroomy got bitten by Golden Freddy.
- According to SMG4: Mario and the VILLAGE, he lives in the Animal Crossing village that Mario visited in SMG4: If Mario Was In Animal Crossing.
- Said former episode reveals that he's married to Alcina Dimitrescu.
- Doomguy neglecting Shroomy could give Bowser's mistreatment of his kids a run for its money.