Secret-Chaser

"Cartman: Aha! Kenny! I should have known! Why do you have a picture of Mysterion in your locker, Kenny?! Unless you are Mysterion!

Stan: Dude, I have a picture of Mysterion in my locker too.

Clyde: Yeah, so do I.

Cartman: What?! Why?

Clyde: I don't know, it's just a cool costume."

- South Park, episode "The Coon"

A standard fixture of the Fantastic Comedy: whenever the series concept requires that an extraordinary person Masquerade as ordinary, there is one person around who suspects them, or has already learned about their secret (usually not every little detail, but enough to see them for what they really are). This character is always the Only Sane Man -- Clark Kenting, fortunately, still works for everyone else -- making this character only look crazy if they try to expose the protagonist.

If the character himself is not convinced that they are just hallucinating -- and some of them are -- they become set on getting proof to show everyone else that they were right all along -- only to fail every time. Sometimes the neighbor fancies herself an Amateur Sleuth, going so far as to spy and snoop whenever visiting the "undercover" character. They may even set traps to expose the character's Masquerade. Usually though they have no idea -- or care -- about the trouble they are getting themselves into around the protagonist, so typically the hero will need to save them when they get too involved.

This character is usually a normal human in the vicinity from their civil life who has noticed that something is odd or amiss with them -- a Nosy Neighbor or a Snooping Little Kid. If the masquerading subject is in school, the character is quite likely to be a teacher at that school, or a School Newspaper Newshound. If the show is a Work Com, it can be a co-worker, often a rival or the subject's immediate manager.

Occassionally, the character will have had an obsession with the supernatural before they meet the protagonist. They may be a danger that comes from the outside -- The Hunter, an Inspector Javert or the Intrepid Reporter.

Often there is a character around who could give weight to their claims, an authority to present their proof to who views with bemused indifference their ranting insistence that something is strange about the main character -- a husband if the character is the neighbour, the school principal if they are a teacher, a more senior manager in a Work Com. Sometimes a Sidekick whom they drag into helping them -- usually an underling, such as a school janitor if or an intern -- is also present.

Contrast/compare Secret Keeper. See also Not So Imaginary Friend.

Anime

 * Ah My Goddess: Sayoko noticed how there is something off about Belldandy, and goes to great lengths to uncover her secret. Of course, since Belldandy is a Goddess who Can Not Tell a Lie, she simply tells Sayoko the truth. Sayoko never believes her and continues her schemes.

Comic Books

 * Fables did this twice; once when a reporter thought all the folks of Fabletown were vampires; and planned to "out" them to the world. The second time was with Thorne, later.
 * Ben Urich was this to Daredevil; in both the comic and the movie adaptation.
 * The Silver Age Lois Lane would occasionally act this way around Superman, as would Lana Lang around Superboy.
 * Vicky Vale in Silver Age Batman comics.
 * In the comic PS 238 a school full of kid superheroes in their civilian secret identity disguises also includes the extremely paranoid Cecil Holmes, who is convinced his school is full of disguised aliens.
 * Semi-subverted in that Ironically, he never manages to spot the actual alien at the school, Prospero.
 * One of these neighbours sicced an occultist on the Fantastic Four in one issue, mistaking them for witches.
 * Tim Drake used his abilities as this to convince Batman that he should become the next Robin.
 * Ultimate Spider-Man played with this. Peter's classmate, Kenny "Kong", was typically displayed as the dumb jock type. One day, while lying in his bedroom, he was able to put together the coincidences and realized Peter was Spidey. Of course, none of his classmates believed him.

Film

 * In The Return of Hanuman, Munni noticed that Maruti looks rather different. The other gods were worried that she will expose Maruti's secret that he is actually Hanuman. Fortunately, she didn't even have the will to figure out what's wrong with Maruti.

Literature

 * Inverted in Harry Potter. The Dursleys know full well that there's a whole world of magical people and creatures hidden from Muggles, and go to absurd lengths to try to pretend it doesn't exist.
 * Robert Audley in Lady Audley's Secret goes to extreme lengths to prove the titular character's secret, as he figures out what it is fairly early on.

Live Action TV

 * Gladys Kravitz on Bewitched.
 * To a certain degree, Dr. Bellows on I Dreamof Jeannie, but more so his wife Amanda.
 * Notably absent in Sabrina the Teenage Witch, much to the surprise of all.
 * Until Brad showed up.
 * Sergeant Doakes from Dexter.
 * Detective Quinn becomes one as he starts to get suspicious about Dexter. However, he has secrets of his own and the closer he gets to the truth the less willing he is to pursue it further.
 * Jack McGee from the TV show The Incredible Hulk.
 * Elton Pope (and the rest of LINDA) from the Doctor Who episode "Love and Monsters".
 * Eugene Jones from the Torchwood episode "Random Shoes" (after his death, Gwen ended up investigating him).
 * One Monty Python's Flying Circus sketch takes this to extremes by having two women spy on their neighbours with high-tech surveillance equipment.
 * In Pushing Daisies,  eventually ends up as one of these.
 * Phil of the Future: Vice Principal Hackett is convinced that there is something odd about the Diffys. But he is only half right about them. He believes them to be space aliens, but they are actually from the future.
 * In the second season of Mighty Morphin Power Rangers, Bulk and Skull became obsessed with discovering the Rangers' true identities.
 * Cassidy and Devin in Power Rangers Dino Thunder were like this too (well, Cassidy chased the secret and dragged Devin along with her). They, like their predecessors, had no idea they were going to school with the heroes
 * Lois Lane in the Live Action TV series The Adventures of Superman.
 * Roger a.k.a. 'Pink-Eye' from Grossology is obsessed with proving that Abby and Ty are Grossology agents and later with becoming one himself.
 * In early seasons of Smallville, both Lex and Lionel Luthor suspect something is different about Clark, and go to great lenghts to try and discover it. Later on, in the episode "Hydro" Lois Lane attempts to prove Oliver Queen is Green Arrow. Oliver and Clark manage to convince her otherwise by having Clark dress up as the Green Arrow. In the episode "Identity" Oliver repays the favour by pretending to be the Red-Blue Blur when Jimmy Olsen suspects Clark Kent.
 * The first season also had reporter Roger Nixon, who Lex hired to investigate Clark Kent.

Video Games

 * The sushi chef seems to be the only one who can see through Octodad's Paper Thin Disguise.
 * Bob Copper in Mega Man Star Force has this as, quite literally, his job description. When he's not trying to find out who Mega Man is, it's because the police force has tasked him to discover the roots of something weirder.

Web Comics

 * Ivan Bezdomny from The Wotch, an intrepid (school) newspaper reporter who was dead set on finding the source of the 'Weirdness' surrounding Tandy Gardens.
 * George from El Goonish Shive, a comicbook shop clerk who works alongside one of the main characters and was insistently badgering said main character to reveal the backstory of a magical incident that took place near the shop. That is until he abruptly dropped the subject after the main character revealed one of George's assumptions to be wrong and revealing to the audience that the reason for his badgering was to learn something that depended on the assumption being correct and that he didn't actually care for the backstory itself after all.

Web Originals

 * Shin from Sailor Nothing -- except that

Western Animation

 * Dib knows that Zim is an alien out to conquer Earth in Invader Zim. Unfortunately, despite constantly trying, he can't convince anybody else (except his sister Gaz, but she doesn't care).
 * Denzel Crocker of The Fairly Odd Parents is constantly out to prove the existence of FAIRY GODPARENTS!.
 * Hans Rotwood from American Dragon Jake Long. A teacher of the titular hero, he has an interest in supernatural creatures and is set on proving that Jake is one to the world once he has found out.
 * Candace from Phineas and Ferb, the older sister of her two Child Prodigy brothers whose crazy inventions only she witnesses: by the time she fetches their mom, it always disappears by some crazy coincidence.
 * Mandy from Totally Spies has acted like this a few times, though the situation is usually solved by Laser Guided Amnesia. She is the Alpha Bitch in the main characters' class who seeks to cross them wherever she can.
 * Lt. Mitch Kellaway in The Mask movie and the animated series is both this and the Inspector Javert. (Not so much in the movie where Stanley does do illegal things with the mask, such as robbing a bank (he just happened to do so just in front of the goons).)
 * Every Chicken Boo segment contains exactly one character who is aware that the titular character is in fact A CHICKEN I TELL YOU! A GIANT CHICKEN! This guy/gal tends to get ridiculed throughout the episode (or sometimes kicked out) until Boo's masquerade is stripped away somehow. If the wolf-crier was sent away, he/she returns at this point to say, "I told you he was a giant chicken."
 * In the South Park episode "The Coon", Cartman spends most of the episode trying to find out who Mysterion is. He finally succeeds in having him unmasked when he convinces him that villains would continue to threaten the town unless he revealed himself. But the audience doesn't find out who it was until a later episode.  Also everyone figures that The Coon is just Bruce Vilanch.
 * In Ultimate Spider Man, Mary Jane Watson, aspiring journalist, seeks to discover Spider-Man's true identity.
 * In Batman the Animated Series, there is a villainous scientist who figures out Batman's secret identity. He tries to tell other villains what it is, even Two-Face who used to be Harvey Dent, Bruce's best friend.