The Fake Cutie

While The Cutie is known for being a character that is sweet, lovable and innocent, some characters, while apparently cute, aren’t as innocent as they seem.

The Fake Cutie is either a full-grown woman or otherwise mature person, but insists on dressing, speaking and acting like a child, sometimes to a sickening degree, or at least much younger than she is in order to be cute and "get dates" or other benefits.

This cutie act is actually camouflage to hide her intelligence, her true self, her nefarious intentions, or her insane jealousy. She tends to be very manipulative and is often dangerous to those that see Beneath the Mask.

She is the cute counterpart of the Stepford Smiler, and may even be her daughter.

Compare Deliberately Cute Child, Kawaiiko, Keet, The Ingenue, Kiddie Kid, The Cutie, One of the Kids and Dawson Casting.

Examples of The Fake Cutie include:

Anime and Manga

Washu on Tenchi Muyo!, though it's definitely an act; she can move in and out of it at a moment's notice (she even physically alters her body to switch between the usual "Cutie" act and something closer to her true age). She's the smartest person in the galaxy, and Really Twenty Thousand Years Old... And that's in the continuities where she's closest to what she appears.
On top of being a Genki Girl, Vandread's Dita fits this description pretty well. She talks like a child ("Hello, Mr. Pilot!") and acts like one, even if you ignore the fact that she plays with toys.
- Actually, her Character Development reveals a more... complex reason why she grew up this way: When she was little, being a Cheerful Child brought hope and smiles to the adults around her in a tough time. She resolved to be as cheerful and cute as possible to continue helping out as best she could, and it kind of stuck. To her credit, she didn't let this get so out-of-proportion that she didn't learn more practical skills, like cooking, sewing, flying a Humongous Mecha and delivering babies - The last skill in particular was instrumental to saving the entire crew in the opener of the second season, as the cry of a newborn baby was the password to unlocking the central computer of the ship from its BSOD.
Miu from Ichigo Mashimaro. Cute is practically a way of life for her. Mind, so is Jerkass.
Yokohama Kaidashi Kikou seems to suggest that, if we ever develop Ridiculously Human Robot Girls, we'll probably intentionally program this trope into them... and they're probably right.
Death Note‍'‍s Near is an eighteen year-old genius who sits around playing with toys all the time and who, when he has to fly to Japan, gets an ally to essentially chaperone him because he's never flown on his own before, then sits on the flight fiddling with a model car. He's not trying to be cute, though - he's just extremely weird.
- Misa is this whenever she has all her memories intact and serves as the second Kira. Otherwise, she's a genuine cutie.
C-ko Kotobuki from Project A-ko.
Komoe Tsukuyomi from A Certain Magical Index. I mean.... Bunny pajamas, and ends her sentences with 'desu'? Who are we kidding here?
- Makie behaves like this in general.
Rena from Higurashi no Naku Koro ni fits this trope, or so it appears. The other main characters too, but they're broken more often.
- A straighter example would be Rika. She spends most of the time looking adorable and acting like a child, much to the joy of Rena (when she's not crazy). But come the second half of the series, we learn that she is in fact quite mature (and somewhat cold), having lived through hundreds of years worth of bloodbaths, at the end of which she is always killed. And her only option is to live through each one, waiting until a miracle occurs or until she cannot resurrect into another timeline.
Akira in Lucky Star, despite being 14 and thus younger than anyone else in the cast, has the personality of a dried up old entertainer completely disillusioned with the industry. So the joke behind her little Lucky Channel segments is the rapid personality switches between a Kawaiiko girl even younger than her real age and her bitter, real personality. The cutesy act apparently doesn't go over well with the audience compared to Minoru or the main cast, of course.
Yaya from Shugo Chara goes as far as having her Shugo Chara, AKA Anthropomorphic Personification of her True Self, be a baby. Her Magical Girl form even wears bunny pajamas and her special attacks include rubber ducks and a giant rattle. Oh and she's prone to burst into tears if hurt.
Biscuit Krueger in Hunter X Hunter is an expert combatant and, for some time, a mentor to the main characters. She has the ability to alter her body to appear like a teenage girl; she does so because her true form is Gonk and she is ashamed of looking like that. She also likes the bonus of keeping a low profile and, if she ever needs to get in a fight, having the opponent go easy on her due to her cuteness.
Flay Alster from Gundam Seed becomes this when she goes Yandere after her dad's death, acting sweet and kind to Kira while planning to get him killed as "punishment" for him not being able to save her old man. Character Development fixes that, however. And then she dies.

Film

Kit ten, from Breakfast On Pluto.
Esther from Orphan fits this perfectly, her entire life is based around hiding her chaotic personality with a cute-girl facade.

Literature

Natasha Rostov tries this for a long time in War and Peace. She's eventually broken though.
Quite a few of David Eddings' novels have the goddess Aphrael. Her "real" form is an adult woman, but she always appears to everyone as a eight year old girl. This allows her to use The Power of Love on everyone. When a cute young girl asks to sit in your lap, no one can refuse. Once she sits in your lap and gives you a kiss, she will get her way on everything she wants.

Live Action TV

Princess Ariel of Wizards and Warriors was the epitome of this trope.
Similarly, Vala Mal Doran of the later seasons of Stargate SG-1 is portrayed as being in her early to mid thirties, but most of the time acts like she's in her early twenties at best, even wearing her hair in a pair of unbraided pigtails. It's almost certainly calculated to make her appear harmless, because in an episode when she loses her memory, aside from trying to continue her thieving and lawless ways, she acts more maturely. It's helped when people make reference to pop culture, as she's not from Earth, allowing her to demonstrate something approximating real naivete.
- And when she stops acting cute and acts competent instead, she unnerves everyone. It's a skill.
- Some fans found Vala intensely annoying due to this immaturity and consider her to be a symbol of the shark-jumpage of SG-1's final seasons.
- Whereas other fans remembered that it's Claudia frickin' Black kicking ass, being hot and hilariously funny, and loved her.
At the beginning of Supernatural, even though he was 26 and slightly darker than your average cutie, Dean was this in a nutshell with his 4-year-old-like pride in his EMF meter and his car and his love of food and girls. But since the breakage of early Season Two, any later attempts to act like this again comes off as more Stepford Smiler than genuinely cute.
Annie on Community is in-universe compared to Pollyanna and Disney Princesses, she is okay with her sexual repression, and other characters have commented they "try not to sexualize her".

Music

Baby Spice of The Spice Girls.
Michishige Sayumi from Morning Musume plays this trope for laughs constantly in her TV appearances, putting on the attitude of a cheerfully narcissistic Kawaiiko.

Newspaper Comics

Nermal from Garfield is actually shown to purposely stunt his own growth to stay a permanently cute kitten.

Theater

A subversion occurs during Murder Mystery Dinner Train, where one of the suspects, a one-time child actress, was revealed to have remained young and cute so long because her agent used a variety of drugs to stunt her growth and keep her in the movies as long as possible.
Rosalie in Oh Dad, Poor Dad, Mama's Hung You in the Closet and I'm Feelin' So Sad is an oversexed version.

Video Games

Momo Karuizawa, the skipping, pigtailed, stripey-socked, 4 1/2-foot tall tennis player from Rival Schools. Oh, and she's also evil. But she gets better in the Gorin path - it turns out a good part of said evil came from her loyalty and love for Kurow, and then he throws her under the bus. Shouma saves her and tells her to pick a side, and this drives Momo to a Heel Face Turn.
Liliana in Princess Waltz. Some of it is natural, some of it is obfuscating stupidity. Hints start dropping fairly early that she's not as young as she looks.
Heart Aino from Arcana Heart.
Vanille from Final Fantasy XIII is a Deconstruction of the Cutie. She pretends to be innocent in order to hide her guilt.
Kohaku from Tsukihime. Her faked personality isn't revealed until you get to the second to last route of the game, though there are hints beforehand. However, it actually appears as though she's been faking her personality on accident and out of habit. As a child, she was something of an Emotionless Girl, but now when Shiki see her he thinks to himself, 'If only Hisui had some of Kohaku's cheer.' *wince*

Web Comics

Portia from My Life In True.
Bailey from Alternate. She even etches smiley faces on her food.
Runie from 200:20 because of her carefree behavior and the fact that it's lead to believe that her behavior is mostly just an act she's putting on for one reason or another.

Web Originals

Less Than Three Comics' Brat Pack features Pixel, who is one year younger than her team-mates, and acts about five years younger most of the time, and has a blast doing so.

Western Animation

Quinn Morgendorfer and her three friends on Daria. Mildly subverted because Quinn is as smart as Daria or nearly so, but rarely applies it or develops her intelligence.
This is spoofed on Drawn Together with Strawberry Sweetcake, a parody of Strawberry Shortcake. She turns out to be a homicidal and kinda-cannibalistic Stepford Smiler.
Mary "Baby Doll" Dahl of Batman: The Animated Series was a child actor with a growth deficiency which prevented her from physically aging, and similarly prevented her career from advancing as well. Years later, she snapped and started kidnapping old cast members, but kept doing so in character as "Daddy's widdle precious". It's only at the end of the episode that she breaks character and reveals what she's really like, in a speech that's a real Tear Jerker.

"Why wouldn't you let me believe...?!"

Harvey from Sabrina the Animated Series is a rare male example, though not all the time.