Nikki Reed (Rosalie Hale): So, Kristen, there must be something really special about you for Robert to take such a liking to you and risk the lives of his entire family. Tell us about yourself. Kristen Stewart (Bella Swan): Me? Oh, no. I'm just a hollow placeholder for all of the teenage girls in the audience to project their personalities onto. I have none of my own whatsoever.
Ryuk, in Death Note, is the character who's in it for the same reason as the audience is: Gambit Pileups are fun to watch.
Kirie serves this purpose in Uzumaki: asking the necessary questions as well as witnessing all the strange goings on in her cursed town; and her love interest, Suichi, plays the role of Author Avatar, providing many of the answers that would have been difficult to provide otherwise.
Medaka Box: Zenkichi Hitoyoshi is quite literally, the Normal of the main cast. He's often left to comment on the absurdity of the cast, but isn't without his own quirks and moments of badassery.
One of the Fullmetal Alchemist OVAs used this, "filmed" in the first person from the perspective of an unnamed probationer alchemist who interacts with Fuhrer King Bradley and Roy Mustang before a giant alchemist vs. homonculus battle.
Robin was introduced to the Batman comic in order to appeal to the young audience who bought the comic. Even among the various Robins, Tim Drake (Robin III) is often cited as the easiest to relate to and identify with as he wasn't and orphan, acrobat, or street rat. Just a regular kid who knew Batman needed a Robin.
Super-Boy Prime is an interesting example. He's from Earth-Prime, which is portrayed as the "real" Earth, our Earth. He was a Kryptonian and the only super-powered person in a world without them, and everything he did in the Multiverse could be read in the comics. He is what happens when you give a bullied kid superpowers and take his world away from him, make him kill so much, then put him back in his world, a world where now, everyone hates him. And to think, he used to be a sweet little kid that read Superman comic books, dreaming that he could be like him....
In Toy Hammer, this role is passed between Michael, Alice and Vincent.
Any reader-insert fanfiction, meaning the main character isn't often given a name and is addressed as "You" in the narrative, and "Your Name" in the dialogue. Amusingly, some of the reader-inserts have more personality than non-reader inserts.
Yeardley Smith is this for the viewer of The Simpsons Movie. In the commentary, she asks questions about shooting techniques and the like that other commentators refer to. She very often does the same in DVD commentaries of the regular series.
Director Bruce Robinson used this trope so literally that the second half of his title duo in Withnail And I doesn't even get a name. Paul McGann's character (credited as "...& I" in the credits, but revealed to be named "Marwood" in the script) is never named in the course of the film, allowing the audience to more easily identify with his misfortunes.
Joe Black in Meet Joe Black, particularly at the beginning (when he serves as the exploratory vehicle within Bill Paxton's estate), and the end, when he tears up watching the party-farewells and acts as the receptacle for Bill's summative reflections - essentially parroting the anticipated reaction of the audience watching the end of the movie.
In his introduction to The Book of Lost Tales, Christopher Tolkien supposes that the reason The Silmarillion was less popular than The Lord of the Rings is that it lacked an Audience Surrogate. In fact, the original draft of The Silmarillion (the Lost Tales) actually did have an Audience Surrogate — a Man named Ălfwine of England to whom the tales of the First Age were narrated by the Elves.
Bella of Twilight has a very inconspicuous personality, her actions are often hard to interpret, her characterization is only implied and the story is told in the first person so the audience can project themselves onto her very easily by disregarding some elements of said characterization when they contradict their views. Stephenie Meyer has even said on her website that she deliberately avoided describing Bella's physical features so that it would be easier for the readers to picture her as themselves.
Firestorm in Less Than Three Comics' Brat Pack. Even though he should be the opposite, what with his family upbringing and all. Sometimes Mr Perfect will take this role.
Italo Calvino's If On A Winters Night A Traveler, written almost entirely in the second person, is centered around two readers: one as a stand-in for male readers, another for female.
To some extent, Taran, the hero of the Chronicles Of Prydain. The author never gives him a physical description, or an age, and his backstory isn't revealed until the final chapter of the final book. Although he is, ultimately, the true hero of the series, he's also something of an Everyman, making it easy for young readers to connect to him.
Mister Rogers' Neighborhood: When Fred Rogers is alone with the camera, he's a parental character. But when he's with a friend, they become the parental figure and Mr. Rogers becomes a child on behalf of the audience.
Penny fills this role for non-geeky fans. Whenever one of the guys makes an obscure reference to something in geek culture, Penny's always there to sarcastically ask what the heck they're talking about, when many viewers were wondering the exact same thing.
For everyone else, it's Leonard, a generic geek without his friends' more overt flaws (Howard's lechery, Raj's gynophobia, and Sheldon's antisocial tendencies).
Jim's mugging for the camera on The Office often reflects how the audience perceives the ridiculous events on screen.
LOST did this a couple times to acknowledge fans' desire for answers. In season 1, Hurley gets frustrated at one point with all the mysterious happenings on the Island, saying that he wants answers. Then, in the epilogue, "The New Man in Charge", Ben comes to visit the guys at the DHARMA packing plant. As he turns to go, one of them says "Wait! You can't just leave without giving us any answers!" which is exactly what the viewers were all thinking at that point.
Similar to the Penny example above, Agent Booth on Bones responds to Bones and the other squints just like any non-anthropologist in the audience would, making them explain the more complicated concepts in laymen's terms and sometimes lampshading their Sesquipedalian Loquaciousness:
Dr. Hodgins: It's seventy percent amorphous silicon dioxide. Booth: What's that? Dr. Hodgins: It's a common domestic container. Booth: Oh, like a jar. Why can't we just say "a jar"?
In one episode towards the end of Season 7 of Supernatural, it is Anviliciously clear that the showrunners really want us to see guest star Charlie as a Type 3, as they have her spend a huge chunk of time making sci-fi references, wondering what Hermione would do in a given situation, and talking about Comic Con (the fact that they cast Felicia Day in the role helps hammer in the point). It feels as if the entire opening sequence with Charlie is basically the writers saying "See, SPN fans? She's just like you! Root for her, dammit!" However, some in the audience felt like the show was trying way too hard to get the audience to like her. And the fact that she was an Anvilicious and strident mouthpiece for the writers' political viewpoints (and the fact that she takes illegal actions on behalf of the writers' viewpoints) meant that it was virtually guaranteed that there would be a chunk of the audience that would see her as annoying rather than as the sympathetic, plucky heroine the writers wanted the audience to see her as.
John Carter of ER was apparently this, as his character was introduced as a 3rd-year medical student, new to the hospital, unlike the other, who were rapidly established as having worked at there for the past 1-5 years with considerable backstory.
In The Insect Play, the Tramp (known as the Vagrant in some translations) is the only human character present for most of the play. He mostly serves to draw analogies between human societies and insect societies.
"Interviewer," in Autistic License. In some variations, he spends the entire time on stage! Kudos to any actors with that level of stamina. Unless the variation is just him sitting the entire time, which only hammers the point home.
Magma in the first X Men Legends game, who is a mutant saved and taken in by the X-Men. We go through her first days in training all the way up to becoming the newest member of the team, though the first few missions don't even have her as a playable character.
Ethan Mars in Heavy Rain. There are four main characters in the story, but it's clear right from the beginning that Ethan's the one the player is supposed to empathize with the most.
Raiden is pretty much this in Metal Gear Solid 2 Sons Of Liberty. Like the player he has experience from the prequel based on "virtual reality" and the game is not subtle at all in later parts when Raiden is told to "Turn the game console off." It is also worthy to say that Raiden has dogtags written by the player in the game's beginning and he throws it away in the end, having decided to find a new identity.
Martin Walker from Spec Ops The Line is played for every negative connotation this trope provides. He treats the events of the story the way your average modern military shooter player would: As a power fantasy and a chance to feel like a hero. In the process, he does a number of horrible things, and every single one of them is your fault.
Tobiah, the teenage narrator of The Graystone Saga, is just an ordinary kid who accidentally gets swept up in the quest of the story's actual protagonist. Not much has been revealed about him, enabling the reader to more easily project themselves into his role.
Ahsoka in Star Wars: The Clone Wars: a child, not having learned yet all she needs in order to survive in the universe, suddenly thrust into a life of excitement and adventure (and, more importantly, authority [at least, in her own mind] over more experienced adults). Isn't that what lots of kids fantasize about (among other things)?
Spike seems to be a surrogate for the Periphery Demographic, being The One Guy and all. Case in point, the end of the episode "The Ticket Master", where he complains that he's not interested in going to the Grand Galloping Gala, but is secretly delighted about getting an invitation.
Later, Big Macintosh takes up the role of representing the Periphery Demographic in "Lesson Zero", where he, despite being the most muscular pony on the show at the time, shows genuine interest in owning an old doll meant for girls. Sound familiar?
The Cutie Mark Crusaders represent the children who aren't sure what they're good at and what they want to do with their lives.
And of course Twilight Sparkle is this for the intended audience demographic.
Twilight Sparkle: All the ponies in this town are crazy!
Beast Boy acts as this sometimes in Teen Titans. As the youngest, and the least smart, he sometimes has the science-y stuff explained to him by his more educated teammates (e.g. the Chromaton Detonator in Apprentice: Part One, Xenothium in X).
Kid Flash in Young Justice. He is the only member of the team with a remotely normal childhood, as well as the only one who lives in an a two-parent household and attends public school. Rocket takes on this role later in the series, where other character summarize the events of past episodes for her.
Lisa, Yeardley Smith's character, often fills this role on the series (whenever Comic Book Guy isn't around):
Marge: Don't you remember when Maggie shot Mr. Burns? Homer: I thought Smithers did it. Lisa: (under her breath) That would have made a lot more sense.
Frank Grimes in the infamous eighth-season episode "Homer's Enemy". The character's sole purpose was to represent a realistic person from our universe — accustomed to toil, pressures and hardship with little, if anything, to show for it — transplanted into a universe that caters to and rewards the lazy and stupid, and how it would understandably drive him/her absolutely insane.
Gus in Recess can be considered one, as he seems the most confused about the way the school is set up, causing the other kids to explain them to him- and the audience.
Fry in the first season of Futurama, although he started to move away from this role once he became more accustomed to life in the 31st century.