This trope defines a character who is interested in fantasy, sci-fi, computer stuff, or other manner of geeky pastimes. The difference between this character and a proud geek is their total lack of embracing their geeky nature. To a vast majority of the population, they appear normal, but a certain group of friends is privy to their true nature.
Reasons for covering up their geekiness may stem from a perceived image of themselves they desire to maintain or running with a group of friends who aren't normally interested in that kind of stuff. Expect Freudian Slips when their topics of interest are discussed, and cries of Or So I Heard in a vain attempt to cover it up.
Ogiue takes this to rabid levels, telling everyone how she HATES otakus. Strangly enough, it's completely justfied, complete with probably the most disturbingly realistic Freudian Excuse ever seen in manga.
Sana in Myself ; Yourself. He has an entire box of full of anime stuff (including an AIR artbook) which he hides from his friends. He also plays visual novels on his laptop in his spare time.
Yua in Chaos;Head. Whilst appearing to be a normal honor student, she keeps up with the latest anime and starts to buy the collectible figures, whilst maintaining her flawless image. However, her geek side may be a ploy to earn Takumi's trust.
Kirino Kosaka from Oreimo. She's the person you would expect the least to have anime - and much less, eroge.
Izumi Wakase, one of the post-anime characters from Lucky Star.
Ben Clouseau of Full Metal Panic! is a secret anime nerd, as we find out in the OVA. He tries to keep his DVD stash a secret, but Kurtz finds them and sows havoc (read: extensively taunting him then corrupting the videos).
Chisame from Mahou Sensei Negima! is a huge otaku and an online Idol in secret, but denies all of this in the real world due to overwhelming embarrassment about her hobbies.
Yayoi Kise from Smile Pretty Cure! is an otaku who dreams of being a manga artist, but hides it for fear of being teased.
Jotaro Kujo from JoJo's Bizarre Adventure is a stoic, tough-as nails badass delinquent... who reads technical books about boats and planes for fun, developed his Genius Bruiser tendencies by obsessively watching Columbo, and whose favorite movie of all time is nature film/fictionalized biopic about a guy studying caribou.
Sam the weather girl in Cloudy with a Chance of Meatballs is obsessed with meteorology, and becomes the weather girl to make it seem perfectly natural. Develops a habit of saying something really smart and then backing away from the statement to try not to sound too clever due to a history of being teased for her "nerdiness" as a child.
In Airheads, the LAPD digs up dirt on would-be rock star Chazz Darvey, who has taken a radio station hostage. SWAT team leader Carl Mace reveals to the crowd (that has come to cheer Chazz and his band on) that Chazz is actually Chester Oglivue. Chazz admits that he was a nerd in high school, and tells his girlfriend that he understands if she doesn't love him anymore. But members of the crowd start shouting that they were nerds in high school, too, and the FBI's gambit fails. To make things more funnier, one of those said people is Lemmy from Motörhead.
Frozen: Elsa. According to one of the book adaptations, she loves geometry. This is implied in the movie when she creates her ice castle. (It even gets a shout out in the lyrics of her character-defining song. Where else would she learn the phrase 'swirling fractals'?)
In the Aunt Dimity series, retiree George Wetherhead is initially presented as this over his model trains; he's so shy and retiring about his hobby that when Lori first comes to his door, he seizes her and pulls her inside his cottage so as to avoid having the impending train whistle blast audibly outside his home. Eventually, he is persuaded to show his collection to paying customers, first for a village festival and later on a scheduled basis.
In the second book of the Into the Looking Glass series, the viewpoint US Marine character is this until outright asked about his knowledge of science fiction, it having been decided that such knowledge might be useful for the crew of humanity's first starship. The implication is that many of the personnel aboard the Vorpal Blade were closet geeks until they were thrown together.
Live Action TV
On 30 Rock Jack had a cookie jar collection, which he forced himself to get rid of in order to be able to climb the corporate ladder.
Colin Fisher, one of the rotating set of interns on Bones, actually admitted to being a closet sci-fi geek.
Given their line of work, however, did this really come as a shock to anyone?
Freaks and Geeks has Alan, a bully who picks on Sam, Neil, and Bill; after hospitalizing Bill in a prank gone awry, he reveals that the reason he picks on them is because he secretly likes science-fiction and comic books. Bill even invites him to an upcoming convention, but Alan chooses not to go.
Freddie Prinze, Jr.'s character on Psych, who, after being a not-so-closet geek in high school, has grown up to be, well, Freddie Prinze, Jr., but still has a secret room (even from his wife) of nerd paraphernalia. Naturally, they had to make a "closet nerd" joke when the room was first revealed. Also naturally, his wife turned out to be a closet geek as well.
Jack O'Neill from obfuscates stupidity about science, but he's an amateur astronomer; he denies liking "that sci fi stuff" and claims to have never seen Star Wars. Yet it's him who suggests the name Enterprise for their first big ship, and on a mission in the X302s he wants his code name to be Red Leader.
Also, from Season 6's Episode "Redemption", while going through an X302 Pre-Flight Checklist:
Carter: Navigation? O'Neill: Check. Carter: Oxygen, pressure, temperature control? O'Neill: All check. Carter: Inertial Dampeners? O'Neill: Cool!... and check. Carter: Engines? O'Neill: All Check. Phasers? Carter: Sorry sir.
Spike. While the Scoobies are holding him hostage in Season 4, his biggest concern is missing the next episode of Passions.
Penny is *NOT* this on The Big Bang Theory...in the first season. But after spending time with the guys she's slowly become this, able to recall Star Trek plots (and use them as metaphors), kick ass at video games, and quote Star Wars. But of course she's not going to admit there's a difference between Star Wars and Star Trek to her (very smart, but not into fandoms) friends. Nevermind that she saw the new Star Trek movie on her own and of her own accord before Sheldon did. Although granted there's still a lot of fandoms she really genuinely does not have much interest in (like anything to do with comics books). Also Leonard whom is usually a proud geek attempted this once, it failed.
Penny: What are you doing?
Leonard: No, don't tell her
Sheldon: Playing Klingon Boggle.
Howard: Oh come on, like she doesn't know we're geeks.
The 6th season has Penny not only get into comics, but drag Bernadette and Amy down with her in a battle over the exact mechanics of Thor's hammer.
Star Trek: The Next Generation has an unexpected example in Captain Picard, diplomat, badass, and very serious man, who much to his officers' amusement had a huge geek-out over the realism of the "Dixon Hill" holodeck program. Picard also gets really excited about archaeology, and on the away team exploring an old ship he behaved like a child in a chocolate factory.
Supernatural's Dean Winchester. He teases Sam for being a geek, but he drops references to Star Trek, Lord of the Rings, The X-Files, amongst others. Especially apparent in LARP and the Real Girl, when Sam pulls up a video of the LARP group's activities, Dean comments it looks awesome, but turns stoic when Sam turns to look at him.
Lexi Reed from A.N.T. Farm. Her Attention Whore tendencies came to the fore in the Season Three premiere, when she felt so jealous that she applied for the ANT Farm boarding school; however, she still prefers to be the resident Lovable Alpha Bitch.
Fuuka Yamagishi from Persona 3 is initially one of these, before the protagonist helps her accept this.
In the Lair of the Shadow Broker DLC for Mass Effect 2, Legion is revealed to be a gamer on its own time, having quite an extensive record of achievements.
Rozalin in Disgaea 2: Cursed Memories is shown to be a Prism Rangers fangirl. She gets annoyed when the Rangers membership lacks all 7 colors of the rainbow, and applauds them when fill the roster later on. Then she becomes jealous when Adell is dubbed 'Prism Black'.
Mao from Disgaea 3 Absenceof Justice claims that all the manga, anime, and comics he consumes about superheroes is merely research so that he can discover the source of a hero's power and use it to overthrow his father, but really he's a much bigger fanboy than he lets on.
Joe from It's Walky!. He acts like a macho man but hides an inner brainiac. His profile even calls him a closet nerd.
Billie from "Dumbingof Age" might be a former cheerleader who likes to call people NERDS but she uses phrases like "life-force" in casual conversation. The glasses and somewhat chubby physique don't help either...
In The Wotch, one of the Jerk Jocks that Anne transforms into the Cheer! Squad turns out to have been a closet geek, playing D&D with the local nerd-squad... who were sworn to secrecy on penalty of severe beatings. His new female form, Lita Harper, is somewhat more open and outgoing about her geeky interests, which includes both D&D and World of Warcraft.
Shanna from Fans! spends much of the early part of the series denying her geek-ness. Justified in this case because obsessive fangirl behavior was one of the early symptoms of her mother's mental breakdown; Shanna's afraid that her own fangirl tendencies are signs that she's going insane, too.
Mark of Weregeek is literally the 'were'geek of the title. When the full moon rises, he gets more interested in geeky hobbies. He hides it from his co-workers and girlfriend for fear of being laughed at, but gradually starts to feel interested in the geeky things on a more regular basis. Eventually, he is simply an outright closet geek rather than responding to the full moon.
Even though the mask slips ridiculously often, The Nostalgia Chick will never embrace the nerdy fangirl within.
Ask That Guy with the Glasses makes constant references to geeky stuff, has written fanfiction and prefers instead to mock the living hell out of nerds.
Carl Copenhagan from Demo Reel mocks Donnie and Uncle Yo for their dorky con bromance, but knows enough about Pokémon to make sexual puns about it to sexy cosplayers.
4chan refers to this trope as "hiding your power level". The general opinion is that your power level must be hidden at all costs, and nobody must know that you have such "shameful" interests as anime, video games, comic books, etc. Overlaps with Even Nerds Have Standards, as power level hiders often look down on and berate those who enjoy their hobbies openly, and even have "cringe" threads devoted to doing so.
In 6Teen Chrissy is a big Star Wars geek but hides it from her popular friends, "The Clones", who are three Alpha Bitches who act and dress exactly the same. She'd always been the most independent of them.
On Phineas and Ferb, Candace attends a sci-fi/fantasy convention to get merchandise from her favorite childhood cartoon, but is terrified of having anybody know about it.
Whatever so-called bully Buford says about being there to beat up nerds, the fact is he was at that same convention. In Cosplay gear.
Trixie Tang from The Fairly OddParents loves comic books, but hides it because the other popular kids would probably reject her if they knew.
John Stewart's Justice League cartoon incarnation. As an adult, he's a no-nonsense former marine, but as a kid he was an avid comic book reader. When a dimension-hop allows him to meet his childhood heroes, he admits to a bemused Flash that if he hadn't had those stories as a kid he might not have ended up wearing the Green Lantern Ring.
When he, Batman, Superman, and Wonder Woman are changed into pre-teens, he uses the power ring to create giant mecha-armour for himself.