What would happen if you replaced a Mary Sue with a well-written Ridiculously Average Guy without changing the plot? This trope is a character who gets involved in all sorts of craziness in defiance of all reasonable logic. Basically, they can't even take a vacation without becoming The Chosen One or saving the world. Whereas to a Mary Sue, it's all in a day's work, this character often finds all the craziness surrounding them annoying and must rely on the remainder of the cast to keep their life from becoming an even worse disaster than it already is.
Often, the character has little interest in ever becoming a hero and is basically happy being a Ridiculously Average Guy as long as they get a better job / win the contest / get a date with the hottie / etc. Typically, the character is either a Kid Hero who would rather play video games, or a Deadpan Snarker who has become slightly jaded and Genre Savvy after saving the world 12 times last year. Often Played for Laughs.
See also Weirdness Magnet and It Began with a Twist of Fate. Compare Action Survivor. If the character seems more likely as villain than hero, but ends up on the side of good anyway, they might be a Nominal Hero. If the hero is 'unlikely' due to not being selected by destiny but heroing on anyway, they are The Unchosen One.
Compare The Real Heroes where ordinary people deserves credit for their good deeds.
- Ash Ketchum from Pokémon. He has come back from the dead thrice, been The Chosen One of countless legends, is one of the few that can manipulate Aura, saved the world countless times, and has helped more people than you could possibly imagine. There's like maybe three people outside of his friends who respect him in some fashion.
- Kyon from Haruhi Suzumiya. Justified by a series of cosmic events so ridiculous it's pretty much a parody of the stereotypical Chosen One. And by "ridiculous cosmic events" I mean God wants to get in his pants, or if you take it to WMG he IS God.
- Tsugumi Ohba and Takeshi Obata's series Bakuman。 has the two main characters being told that the problem with their manga is the utter lack of special main heroes, and that their series' tend to suffer from average people as heroes.
- All Doraemon movies have the Five-Man Band (or just Nobita) doing something just to satisfy their wants with Doraemon's gadgets. Thus making them end up having to save the world or Plot Device creatures countless times.
- Most of the Sin City heroes are often on the lower rungs of society and try to live normal lives. They just keep getting wrapped up in the craziness of the city against their wills.
- Rick Jones from Marvel Comics has elements of this. He is more of an Unlikely Sidekick, though. He's just a normal guy who has ended up being the sidekick to a number of heroes from the Hulk to Captain America, usually because he stumbled onto the wrong place at the wrong time.
- Then he becomes a superhero of his own right as A-Bomb.
- Scott Pilgrim. The fact that he was played by Michael Cera in the movie doesn't help either.
- Joe in Joe Versus the Volcano. 
- In The Relic movie, Margo's just a scientist... who manages to kill the monster. (In the book, Penderghast—a Marty Stu albeit a likable one—kills it unspectacularly.)
- Another example: Detroit Rock City has four pseudo-stoner losers spend the first half getting into trouble, then the second half just getting the favor of God itself as they luck out on everything.
- Ash in the original The Evil Dead (1981) is all over this trope. He works in the hardware department of S-Mart and is just a regular guy. By the end of the film, he's in near-hysterics and is covered in mud, blood, and his own excrement. Of course, in the sequels, this changes dramatically.
- Lionel in Braindead/Dead Alive is a great example of this trope. He starts out the movie as a timid man living with his mother, and ends up dispatching all the zombies with a lawnmower.
- Carl Frederickson and Russel from Pixar's Up. Just try explaining them someone who's never seen the film. One's an elderly septuagenarian, the other is an overly enthusiastic boy scout who's never been to the wild, and together they fight evil in Venezuela.
- Shaun in Shaun of the Dead never wanted to be a hero saving his loved ones from the Zombie Apocalypse, he just seemed to be the one could could deal with it pragmatically.
- Joe from Idiocracy. As is stated, he is average in every way.
- Also from a Simon Pegg movie, The World's End's Gary King is an unemployed, suicidally depressed Manchild trying to relive the glory days of his youth by going on a drunken bender with his former friends. He and his friends (well, two of them) also fight off and survive a horde of body snatching robot duplicates and he's eventually the one to tell the Network to fuck off an leave humanity to its own devices. All of this during the aforementioned bender. He becomes a more conventional hero After the End by fighting for the now discriminated-against Blank versions of his friends.
- Caspian Knoll of Astral Dawn is this. Even he doubts he could be that important until he rises to the occasion.
- Rincewind in Discworld is basically this. All he wants is to be bored and eat potatoes, and all he has to help him is some narrative savvy, a knack for languages and running away, and The Luggage. But he's basically been chosen as the hero of the Disc.
- Tavi in Codex Alera. The whole series started because he was looking for some lost sheep, though eventually it becomes apparent that he was actually the heir apparent to the entirety of Alera.
- Bilbo Baggins. Possibly the ur-example.
- His nephew Frodo, to a lesser extent. (Of course, this was probably the whole point. Gandalf figured that Frodo was beneath notice to Sauron, and could successfuly fulfill the most vital mission. This may, in fact, be why he wanted to recruit Bilbo in the first place, to earn the trust of the folk of the Shire again.)
- The title character of Farmer Giles of Ham.
- Richard of the Sword of Truth considers himself this for the first book and part of the second. He finally accepts his powers and heritage, and the responsibilities that come with them.
- In Those That Wake and its sequel, most of the main characters are this, particularly Mike, Laura, and Rose.
- Night Man: Johnny Domino appears to be one to outsiders, though even before he gets his powers he's an expert in martial arts.
Future Victim: I will not place my life in the hands of a ... a saxophone player!
- Sir Percival was originally the meekest and most naive of King Arthur's knights, having gained repect gradually through several misadventures. Eventually, he is, in one version of the Grail Legend, one of two knights (along with Galahad) to find the Grail. (Unfortunately, most versions after that tend to leave him out, only giving credit to Galahad.)
- Gordon Freeman of Half-Life. Just an average everyday theoretical physicist until that fateful day, he ended up fighting off hordes of aliens while beating back those pesky black ops trying to keep him quiet, then goes into the other dimension to fight the creatures on their own turf, culminating in defeating their all-powerful overlord to prevent the entire earth from being invaded by the things. And all this with a magical satchel of fairly common weaponry with the occasional science fiction import.
- U-1 of Gitaroo Man is a really unlikely hero. Basically, he came back to his house after getting beaten up, which prompts his dog to teach him how to Gitaroo. Right after, a demon with an axe threatens to chop U-1 in half, and the next thing you know, he's actually from a bloodline that automatically makes him the Chosen One for a faraway planet. I don't get it either, just go with it.
- Harry Mason of Silent Hill is an example. A writer in his early thirties is thrown into a situation of trying to find his loved one after a car crash by searching the creepy town from Hell which is full of monsters. The manual even states that he has no experience of using a firearm.
- All of the main Silent Hill series games invoke this trope with two exceptions. Harry Mason is followed by James Sunderland in Silent Hill 2, a clerk. Heather in Silent Hill 3 is a teenage girl with no particular skills. Henry Townshend of Silent Hill 4 is probably a photographer. Travis of Silent Hill: Origins is just a truck driver. Silent Hill: Homecoming introduces the first possible action-oriented hero, ex-Spec Ops soldier Alex Shepard, except Alex is an insane teen or young man who is deluded about being a Special Forces soldier, and the game gets most of the military details right except when it comes to Alex acting and looking like a green beret. Silent Hill: Shattered Memories brings back Harry Mason, and Silent Hill: Downpour introduces our second possible exception to the rule, Murphy Pendleton, an escaped violent convict. So to wrap up, we have a clerk, a photographer, a truck driver, a schizophrenic young adult, a convict, a writer, and a teenage girl, and most of them Punch Out Cthulhu at least once.
- They weren't the true heroes of the game, but in Pokémon Colosseum the Kids Grid were a small organization of hackers that secretly opposed the Ciphers and acted as Wes' primary source of information in the game. Because they were only children, the Ciphers mistakenly believed them insignificant, and left them alone, which proved a mistake.
- In Bad Day LA, the protagonist is a homeless man that was once a Hollywood agent. He was just trying to survive when shit hit the fan in Los Angeles.
- Torg from Sluggy Freelance is just a normal (if kind of ditzy) guy, but just keeps stumbling (sometimes literally) into situations where demon lords or Mad Scientists or supervillains are threatening him, his friends, or the world at large.
- The title character of The Inexplicable Adventures of Bob! is described as the World's Most Average Man who has suddenly become a Weirdness Magnet. He has proven himself a hero repeatedly, mostly just by having courage, compassion, and integrity rather than any obvious action hero traits.
- The Order of the Stick. Seriously. These guys are all mid-level, mildly incompetent half of the time, make dubious after dubious strategic choice, break every doomsday device they encounter, and they get over-shadowed by any paladin or arch-nemesis that crosses their way. How come they have their own webcomic, and it's such a hit? Lampshaded in comic #901 by the Monster in the Darkness even, when trying to convince Xykon not kill the people he recognizes as O-Chul's friends.
MitD: Think about it. Which of these sounds like the hero you need to worry about: The last paladin of a conquered city, beaten but never broken, sworn to stop the evil lich who wiped out his holy order — or some random fighter guy you already snuffed once?
- Ayla Goodkind of the Whateley Universe. He was just supposed to be the Sheltered Aristocrat who would grow up to run the biggest Mega Corp. in the world. He wasn't supposed to get superpowers. He wasn't supposed to become part of Team Kimba. He wasn't supposed to be a Weirdness Magnet who can't let Superhero School Whateley Academy without running into a supervillain. Or a flock of them.
- Droopy, in about half the Tex Avery shorts in which he appears, plays the Designated Hero to Red, who's often victimized by Wolfie, Butch or Spike. Then they wind up pissing Droopy off...
- Fry in Futurama.
- Coop in Megas XLR is just a fat slacker who obsessively plays video games and works on his automotive craftsmanship. He also happened to stumble upon the titular Humongous Mecha at a junkyard and got it for a steal, modifying it to work with his car. All the while, he remained oblivious about its importance until he finds himself face to face with a girl from the future and is reluctantly chosen to be the hero because only he can pilot it at that point. From there, he mostly regards this role as secondary to eating and playing video games.
- In the Ben 10 franchise, the titular character obtained the Omnitrix by accident. While he uses it for selfish reasons from time to time, his first thought when he got it as a ten year old was that he can be a hero. The Chronic Hero Syndrome continued on from there.
- Timmy Turner in The Fairly OddParents! movie Wishology. He was mistakenly believed to be The Chosen One to defeat The Darkness, but he was able to save the universe by befriending it.