Being Spider-Man's partner comes with its own unique pros and cons. Along with worrying if Peter Parker would make it back from patrol or a big battle with any of his many villains, Mary Jane Watson had to consider that the enemies who knew Peter's true identity posed a threat to his loved ones. While over the years Mary Jane dealt with her own share of villainous encounters, her teenage Ultimate incarnation took it one step further when she was injected with drugs that caused her to transform into a ferocious creature known as the Demogoblin.
In the mainstream Marvel universe, the Demogoblin was the product of a demonic possession of the former Hobgoblin Jason Macendale. The original demon hailed from Limbo and was expelled thanks to the Ghost Rider(s) but maintained the identity of the Demogoblin, a darker, mystical version of the Hobgoblin which believed itself to be on a holy mission to cleanse the world of sinners. Armed with all the powers of the Hobgoblin, including the ability to create hellfire and other mystical abilities, the demonic-looking creature fought Spider-Man, Ghost Rider, Moon Knight, and was among Carnage's crew in the 'Maximum Carnage' event. After years spent appearing in other Marvel story-lines, the Demogoblin recently appeared in the 'Absolute Carnage' story-line.
In the Ultimate Spider-Man series, taking place in a separate Marvel universe, writer Brian Michael Bendis and artist Mark Bagley put Mary Jane and Peter's relationship to the test by putting MJ in danger time and time again. As if getting thrown off the Queensboro bridge by the Green Goblin wasn't enough, the Clone Saga story-line saw Mary Jane get kidnapped by what appeared to be her boyfriend Peter. Awakening at the ruins of Oscorp, Mary Jane finds her kidnapper is a disfigured clone of Peter Parker, designated as the Ultimate version of Kaine, who is obsessed with giving her powers so she will no longer be in danger. Revealing that he injected her with an unspecified amount of the drug called OZ, which gave him and Norman Osborn their powers, Mary Jane transforms into a huge, red, goblin-like character.
Later referred to by Bendis as the Ultimate version of Demogoblin, the transformed Mary Jane attacked Kaine while Peter and other Parker clones met at Oscorp and learned who the creature really was. Attempting to appeal to his ex-girlfriend, the real Peter Parker confronted her which calmed her down enough to transform back into her human form before falling unconscious. Taken to the Baxter Building in hopes of finding a cure, Mary Jane's stress upon waking up in the unfamiliar location triggered another transformation where the Demogoblin fought the Fantastic Four. Only when she saw the face of the Scorpion, another illegal Peter Parker clone who recognized her, did Mary Jane calm down enough to transform back and return to unconsciousness, much to the relief of those trying to help her.
The next day, Peter and Mary Jane reunite where Peter professes his love for her and apologizes again for getting her involved. According to Reed Richards and Dr. Franklin Storm, they believe they've found a cure for the OZ drug, meaning MJ would be back to normal with no side effects or further transformations. Although Bendis and his collaborators always left the door open for Demogoblin's return, Mary Jane Watson had more than enough to deal with regarding fallout from Spider-Man's lifestyle than to also worry about becoming a terrifying, red-haired monster. Being a teenager in the Marvel universe is stressful enough as is.