Doomguy and Isabelle is a fan art trend that started in October of 2019. Pieces tend to feature both characters paired in cute and/or gory situations or changing costumes and jobs. The trend came about as a result of DOOM Eternal's delayed-release to March 20th, 2020, the same day as Animal Crossing: New Horizons.
An early piece of Doomguy and Isabelle art was tweeted by user @mragentstrange on July 9th, 2019, with interest in the pairing dramatically increasing as a result of the two franchises releasing their newest games on the same day. The picture that shows Doomguy teaching Isabelle how to use a gun, scored 2,900 upvotes in the coming months (shown below). The trend was first documented on January 20th, 2020, by user Agent Strange in a thread on ResetEra with various pieces of art linked.
Doomguy teaches cute characters how to use guns (aka Doomdad) is another minor art trend where Doomguy is shown shooting alongside young and cute characters, one of the first examples was uploaded to Deviantart by KelvinHiu, where Doomguy is shown with Mega Man, getting 468 favorites and 6K views on the coming years. Another notable example was done by Twitter user IZRART showing Rapunzel from Tangled, that got 70,200 likes (shown below, center left ). During the middle of 2019 Twitter user, MrAgentStrange started to commission artists to draw Doomguy teaching different characters like Splatoon's inklings, Shantae and Kirby, one compilation of those drawing was shared on July 18th, 2019, getting 14,400 likes in the following months (shown below, center right), after the initial batch more examples were commissioned but the art trend started to spread to other artists, on January 24, 2020, Reddit user Mini_Knight shared an edit replacing Rapunzel with Satania from Gabriel Dropout, said edit managed to get 14,000 upvotes in 10 days (shown below, rightmost)
On December 11th, 2019, Twitter user insidematthieu shared a piece of both characters, hoping on the inclusion of Doomguy in Super Smash Brothers Ultimate, said piece got 11,600 likes in the following months (Shown below, left), on the 24 of the next month the user awr_hey uploaded an image of Isabelle donning Doomguy's armor, the picture scored 10,800 likes in the following days (shown below, center), four days later on January 28th, the user PickledChild shared an image of both characters during a demon-killing spree, getting 31,700 likes (shown below, right). Alongside this PC Gamer and Kotaku later wrote their own articles on the trend, referencing the earlier ResetEra thread, both articles noted the strangely wholesome nature of the pairing; Doomguy, a violent gun-wielding demon killer, and Isabelle, the lovable mayoral assistant.