A character who hunts Mary Sues in other Fan Fic stories to get rid of them. Also, the format of story the Hunter appears in. This genre appears in sheltered corners of many fanfic communities and more openly in some places.
The hunter will use a definition of Mary Sue most often based on Common Mary Sue Traits, or perhaps just select Original Characters who really annoy her. The Original Character (or more rarely, a retrofit of a canon character) will kill, maim, injure, arrest, expose, discredit, "purify", or otherwise remove another Original Character and their influence from an established canon. Think Revenge Fic, but for Mary Sue.
Usually, these stories are done in a style similar to Mystery Science Theater 3000, taking excerpts from some other Fan Fic writer's work and building around it. The tone will usually be cynical and satirical.
While some works with this sort of character are revered, random examples of this genre are not as accepted as normal MSTs. Writers are expected to ask permission to MST another fanfic. Mary Sue Hunters will at best humiliate, and more likely kill and humiliate, the Mary Sues hunted; it's hard to get permission for that, and there is expected to be honor amongst fanfic writers. Thus, these stories tend to get banned from most regular fanfic communities.
Mary Sue Hunters are more tolerated if the author also creates the prey — that is, the Sues hunted. These will usually be Parody Sues by definition; the stories they appear in probably won't exist outside the Hunter fic, if there.
The big problem with this sort of character and this sort of fanfic is He Who Fights Monsters. The most well known kinds of Mary Sues are the ones who warp the reality of their 'verses to the author's own liking. As Mary Sue Hunters exist solely to kill perceived Mary Sues and thus change the world of that "verse" to the author's liking, they are by definition a type of Sue. Also, not all authors can reliably tell if an Original Character is a Mary Sue, but Hunters are almost always presumed to be able to tell — they don't generally say "My God, What Have I Done?" So statistically, they are slightly more likely to be a Mary Sue than an OC without an agenda. Some works in this genre, inc. the Trope Codifier, have recognized this. In spite of this, entertaining Sue Hunters, or ones that target hated fanfics, can gain fans who like the Fix Fic aspect of these stories. Badly-written ones, however, aren't any better than your typical God-Mode Sue Revenge Fic story and may even become targets of Mary Sue Hunter stories themselves.
- In the Yu-Gi-Oh! fanfic Not the Usual Kaiba, Mokuba seems to be turning into one.
- In Naruto Veangance Revelaitons, an ending that the author's former stepbrother wrote has Edfred and his boyfriend Muskoka restore the cast to normal, enabling them to kill the hated main character, who easily fell into several Mary Sue categories.
- The Gintama fic series Gintama Chromatic Chronicles introduces Serizawa Suzume who is known as a "Fan Dumb Exorcist" and leads an OC-only Shinsengumi squad whose only mission is to kill Sues.
- In the Mai-HiME fic Perfection Is Overrated, the main cast goes up against a group of foes called SUEs, who represent typical Mary Sue types, with powers symbolic of the Mary Sues' impact on the plot, such as a fundamentalist Christian with the ability to reduce the effectiveness of her enemies' powers. Miyu accidentally becomes one by killing Akane's Child and resulting in the death of Mariko, whose powers made her extremely popular on campus and enabled her to become Akane's most important person instead of Kazuya.
- In the Sailor Moon fic, Super Sailor Chibimoon Eradicates all Mary Sues!, the author had Chibiusa, with the help of Helios, eradicate various Mary Sue archtypes found in the Sailor Moon fanfic community.
- Limyaael wrote The Game of the Gods, a The Lord of the Rings fanfic that has Morgoth playing a game with Varda where he creates Mary Sues and places them in Middle-Earth while his opponents use logic and the rules of Tolkienian canon to kill them.
- The Redwall fandom used to have these. It was headed by a weasel named Kelaiah and a pine marten.
- The Epitome of Randomness came up with their own interesting take on the genre. Specifically, a whole organization of children dedicated to the field. Introducing the Sue Slayers!
- Narutaki in Kamen Rider Decade is an odd canonical one. Hellbent on making sure Decade does not enter any worlds except the world of Negatives. He does things ranging from rallying the entire world of Riders against him or at one point summoning an makamou to stop him.
- Self-Extraction is a story about a Badass Normal former BattleTech Pilot who joined the Agency to stop the intrusion of the Dream Maker, a person who lets people live out their fantasies in other dimensions which have in some way caused trouble. Sues range from crushing the toughest folks in Street Fighter, attempting to kill Barney the Dinosaur, and becoming an all-powerful demon god just because the person wants to create a CrapsackWorld with several worlds completely empty.
- The webcomic Mary Sue Academy has a group of Mary Sue Hunters as villains (sort of).
- Lynching ITP is a set of Play-by-Post Games which combine this with Mega Crossover, usually directed at the worlds of Twilight or Inheritance Cycle, though it has a tendency to turn into an Ultimate Showdown of Ultimate Destiny with the Sues in the background.
- Anti-Cliché and Mary-Sue Elimination Society is Exactly What It Says on the Tin. Most Sues hunted by the group are from made-up fanfics, but Sues from real fanfics are, at the request of the author who owns them fair game. (The society hinges on the belief that Sues are often Old Shames of whoever first designed them.) Unlike most, the Society offers rehabilitation to captured Sues, and a few members of the Society are reformed Sues themselves.
- The Protectors of the Plot Continuum could be argued to be the Trope Codifier for Mary Sue Hunters, as whether or not they started it, they definitely popularised the idea. Different from others in that it has its own shared universe.
- Dr. Clef (the character, not the author) of the SCP Foundation has a reputation for "decommissioning" Reality Warpers... most of which are terrible Mary Sues written by people who don't know any better.
- He Who Fights Monsters kicked in pretty hard. Once Clef's fellow Sue Hunter, Dr. Kondraki, was found riding The Dreaded SCP-682 like a bull, they knew they had gone too far and began retconing Mary Sues instead of decommissioning them. Clef's writer has also written a sorta-retcon which leaves to the reader's imagination whether Clef's feats of badassery are real or an unremarkable researcher's records corrupted by an SCP-732 outbreak.
- Ensign Sue Must Die - In Ensign 2: The Wrath of Sue, because of the Enterprise's actions in the last one, the crew of the Enterprise (and The Doctor) travel the multiverse to contain the many Mary Sues that spawned in it with Poké Balls. However, it turns out that had been Sue Prime's plan all along, to get all the Sues in one universe and weaken the fabric between dimensions so she could come back.
- In the Ultra-Fan Wiki Metaverse, the rogue Weekee Malleus is dedicated to hunting down the Villain Sue "Lord Shiny" and his counterparts, all of whom were created in various forms by a now-banned troll.
- An important element of the Editors and Meta Quest setting, where the Transpace Guard organization largely serves to keep the canon intact and functional. Though this includes a lot of things, including nonsensical or damaging crossovers, attempted Fix Fics, plunderers, or people who seek to damage a setting, Mary Sues are more or less their routine targets, and are viewed as dangerous rogue elements and dimensional parasites.
- Pony POV Series:
- It has a somewhat unique, dead serious example with the Blank Wolf. As the Guardian Entity of the timeline, it's meant to erase things that shouldn't exist, which includes Mary Sues. As a result, it can No-Sell their OP abilities. However, in this universe, Mary Sues are Played for Drama as creation-threatening Cosmic Horror level beings created by unknowing Shadows Who Make that attempt to hijack existing stories and twist them to revolve around them, and most of its victims are creations of the author for the story including the Big Bad General-Admiral Makarov rather than ones he perceived as Sues.
- However, not all of the Sues are made by the Shadows Who Make (who are basically the embodiments of the fan communities' aspect of creation). Makarov is revealed to have been a living storybook character created by Pandora, the Token Good Teammate of the Draconequus (and Discord's sister.) It was all a complete accident and she helped put down her creation (who was in their prior form, an alicorn princess.) She sealed it Pandora's Box and things were okay... until the Hooviets got their hands on it.
- Furthermore, the Blank Wolf is by no means perfect at his job per se. For the longest while, it was stalking Shining Armor.... Turns out Shining Armor is the split between two timelines (between a good and bad ending basically) and he is basically living his life to reach up to that point. From a cosmic stance, he was created by the Alicorn version of Twilight Sparkle and embodiment of Hopes, Dreams and Happy Endings. Shining does lure the Wolf to defeat Makarov, but he is still hunted until the Wolf is put down by Cadance, an Alicorn as well. He does come back as a pup to help the CMC during Discord's endgame (where he is named "Blanky.)
- Pinkie Pie packs an Anti-Mary Sue bazooka. In several fights, Rainbow Dash asks her to use it, but she claims it only works on Mary Sues.