They are cute, sweet, innocent and extremely huggable. Incidentally they are also varying shades of violent, unstable, and downright insane.
Cute and Psycho is a trope that describes characters who are genuinely cute in both appearance and mannerisms but have a completely batshit crazy side. Sometimes there are distinctly different sides which may be shown equally, but other times they are mostly one or the other, the Killer Rabbit displaying moments of sweetness and relative-sanity or The Cutie showing hints of a dark psychotic nature. Often there is some kind of Dark and Troubled Past, or Split Personality to justify how the two aspects of the person can both be genuine, but other times no explanation is revealed.
The primary difference between this trope and the Yandere is that the Cute and Psycho character is not driven by an obsessive need to possess a friend or lover. Their motivation, if they have one, can vary immensely. They also don't necessarily have to be provoked to enter their Psycho-state, but can switch for reasons observers would be hard-pressed to determine.
Cute and Psycho is a Sister Trope to Killer Rabbit, Yandere and Enfant Terrible and closely related to Psychopathic Manchild and Beware the Nice Ones. If the "cute" part isn't real, then the character is The Fake Cutie instead. Characters of this type tend to be female, though male examples do exist.
In some anime fandoms this character is referred to as a yangire, an informal fanspeak term. It's also used to refer to Axe Crazy versions of The Fake Cutie. Not to be confused with fangire, which is a species of monster vampire.
Examples:
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Anime & Manga
Ran of Urusei Yatsura is the poster child for (and likely the inspiration for all other) Cute and Psycho characters. On the outside, she's a cute redheaded girl who likes frilly clothing, baking, and flirting with Ataru. In reality, she's an embittered childhood rival of Lum who is out for some very violent revenge on the girl for … well, a lot of things that aren't Lum's fault. Unfortunately, she is often mistaken for a Yandere.
Cilan from Pokémon, probably. At first it appeared as though his burst came from his battle with Burgundy, but in later episodes he starts slipping into this more and more. His flip-outs are mostly played for laughs, though. Except for episode 69 where he flips his shit at Skyla because he didn't like her attitude problem as well as her not taking her job as a Gym Leader seriously.
In Katanagatari, (Warning, this spoils one of the best reveals of the show) Nanami "My little brother kept chewing his nails, no matter how often I told him to stop. One day I ripped out his nails. He never bit his nails again." The ninja she is torturing starts to sweat and shiver. "It's only a story about child discipline."
Shou Tucker. He seems like a sweet, caring dad, but in reality is insane enough to fuse his own daughter and pet dog into a chimera.
Wrath from the 2003 anime version. He starts off as naively adorable, but upon being awakened... Yikes.
Mari Illustrious Makinami from Rebuild of Evangelion 2.0. She only appears for roughly fifteen minutes in the movie, but these fifteen minutes show a rather nice character. As long as she isn't in an Evangelion, that is. When fighting, Mari is absolutely crazy - it is strongly implied that she actually fights simply because she enjoys fighting.
Gundam has a long history of psychotic cute girls:
Four Murasame and Rosamia Badam from Zeta Gundam, both of whom were some of the earliest Cyber-Newtypes and completely insane because of it. Not evil, just frightened and confused girls with a warped sense of reality (it is not coincidence that they pilot something called the Psyco Gundam). Then Gundam ZZ introduces Elpeo Puru and her clones. Puru herself is mostly harmless but is still a Child Soldier with a deep-seated obsession with Judau, while her clones were all mentally conditioned to serve "Master Glemy" and were coldly classified by Zeon as equipment rather than people.
Mobile Suit Victory Gundam arguably has Katejina Loos, who doesn't look especially bad at first - she's initially rough around the edges, but fundamentally good-natured, until Chronicle gets his mitts on her (and main character Uso even states she's changed since joining Zanscare and used to be nice and kind deep down). To further complicate things, in the novels it's strongly implied she underwent Cyber-Newtype treatments, which are known across the series for making those who undergo them violently unstable.
Ghinias Saharin from Mobile Suit Gundam: The 08th MS Team starts as a WorkaholicHot Scientist, but as his health gets worse and his Apsalus project is cancelled, he becomes more and more unhinged. Granted depending on how much you factor his clinginess towards his sister and the aforementioned project into the cause of his breakdown, he might count as a somewhat unusual manifestation of the other thing instead.
Mobile Suit Gundam SEED Destiny has Stella Louisier, one of the Extended members of Phantom Pain, and the series' version of the aforementioned Four. Adorable, childlike, happy... and a supersoldier with mental conditioning and enhanced physical skills from a complex, long-term treatment of drugs (which she can't survive without). She ends up torching half of Europe with a superweapon not out of malice... but because she was scared and had been told that the people there were "bad things" that were going to get her.
Gundam 00 features Throne Drei's pilot Nena Trinity, a Genki Girl redhead who likes to wink, mock-fight with her violently overprotective brother and fellow Throne Michael and throw up the V-sign. She also gives Setsuna his Sacred First Kiss (also the first kiss for the series), much to his disgust. Exactly one episode later, Nena blows up a civilian wedding while on her way to the next mission to vent her frustration with having a hard day of work while others have fun. When asked by her brothers why she fired on civilians, Nena simply giggled and replied, "Heehee, sorry, I pressed the wrong button." In the second season, she dropped the facade as Liu Mei belittled her, and Liu Mei learns the hard way about double-crossing people when Nena traps her inside her ship as it's exploding, cackling maniacally all the while.
Parodied in Galaxy Angel with the Character Exaggeration of Chitose, who already had some belonging issues; when the Angels left her behind or otherwise slighted her without knowing it, she went absolutely crazy with revenge, once even trying to turn them into rice and eat them. (Don't ask.)
Played for Laughs with Kafuka Fuura of Sayonara, Zetsubou-Sensei. This eternally happy, cheerful girl has an obvious and horrible past, involving suicidal parents, relatives in jail, ostracism at school, and at least one exorcism, and yet there she is, enjoying life... for a value of it. She shows no signs yet of snapping, though people are scared of her and fear the day she does. On one occasion, her classmate, resident Little Miss Snarker Meru Otonashi, tries to "read" what Kafuka is communicating through her eyes at her prompting; she sees nothing but a red screen with the words "die die die die die killy kill die die die" and so forth, despite Kafuka never once dropping her happy appearance.
Virtually the entire cast has some sort of vicious or crippling psychosis — it's just that the fifty percent of them who aren't capable of hiding it make the rest look well-adjusted by comparison.
Minatsuki "Hummingbird" Takami in Deadman Wonderland appears to be a shy, sweet young girl with a Freudian Excuse (actually Parental Abandonment Issues) who was imprisoned in the eponymous prison-slash-themepark, mainly due to her having blood-based killing powers called the "Branch of Sin"; the real reason she should've been sent to prison (see next spoiler) is apparently ignored. This persona turns out to be an act to seduce her naive opponent into not fighting her. In reality she is a sadist who not only gets extremely turned on by the suffering of her opponent but she also killed her mother after she panicked and abandoned her in an earthquake and got her brother to kill her father after making it appear that their father tried to rape her. It shows in the dub: initially, she's surrounded by flowers and has a cutesy voice while her fighting side ignores the cuteness and goes all Blood Knightwith manyF-bombs ensued.
There's also Shiro, the hyperactive, scatter-brained apparent Manic Pixie Dream Girl who can and will rip off people's heads with her bare hands whilst smiling like a kid who's just left the sweet shop. She's also the 'Red Man', the unstoppable killing machine who slaughtered Ganta's entire class at the start of the story.
Russia from Axis Powers Hetalia is a mixture of Cute And Psycho and Yandere. He cracked for non-romantic reasons (the Bloody Sunday), but his unstable side comes out especially strongly in regards to Lithuania, whom he seems to have feelings for.
Lithuania's younger brother Latvia was seen as a potential Cute and Psycho, having been abused so much by Russia that he comes close to snapping in the "Lily of the Valley" incident. It's not mentioned later.
Akira Sakura from Narutaru is ordinarily a cripplingly shy, suicidally depressed girl who hates hurting other people, even to the extent where she's reluctant to hit her sparring partner in a martial arts class. This is, however, also the same disturbed girl who eventually stabs her own father to death, calling him out for the sexual abuse she's implied to have suffered at his hands. Furthermore, in an early chapter it's subtly implied she tried to kill her own shadow dragon, just because she wants to be (relatively) normal.
Tsukuyomi of Mahou Sensei Negima! is like this in her earlier appearances. As of her later appearance, she dropped the facade and become a full blown Yandere.
Chizuru Naba seems to be a somewhat less scary example. She is genuinely kind and protective, but there are some scary layers underneath... which are Played for Laughs.
Ryouko Asakura in Haruhi Suzumiya, who manages to remain Cute while being Psycho. However, the human version of her from Disappearance is your standard yandere.
Akira Kogami from Lucky Star, pretends to be a very childish and loveable but can change to a chain-smoking, physically-abusive brat within seconds.
Nearly every Diclonius in Elfen Lied, with Lucy also qualifying as a Yandere.
Chloe from Noir smacks of this, possibly going full-blown Yanderefor Kirika by the finale
The world of Kurohime has one in the form of Yashahime. She's a delicately beautiful young woman whose sworn to avenge the death of her lover, the Grim Reaper, caused by the eponymous heroine. Fair enough, but the reason she "loved" the Grim Reaper so much is he was actively thinking about killing everything in sight, a sentiment she could get down with. In fact, the only reason she hasn't is BECAUSE she want to kill Kurohime FIRST. Compounding all of this is the fact that she is the series' earth goddess, meaning she could kill literally everybody on the planet.
Dai Mahou Touge is what happens when this trope is applied to an entire universe.
Played for laughs with Otae from Gintama: while she seems to be settled firmly on the Tsundere side of the fence, piss her off enough (or simply exist, if you're unlucky enough to be Kondou) and she'll wallop you straight into the ground.
Anemone from Eureka Seven. "Melt away! Melt away! Your brains'll melt away!"
Rip Van Winkle from Hellsing is a little out there, in addition to being a Nazi vampire.
Yumiko Takagi is a sweet, gentle soul who calls herself Yumie and go off her fucking gourd if her glasses are taken off.
Seras Victoria manages to pull this off very well. Her standard persona is a Genki Girl, but get her into a bloodrageand you are in for a REALtreat.
Patti Thompson from Soul Eater. Cute Cloud Cuckoo Lander who exhibits odd moments of manic violence when she feels like it. Really, we should expect this, considering she grew up on the streets with her sister, surviving by mugging random people. Her 'training' suggests she's a mad, violent thing whose skill in combat has been overshadowed (as is often the case for Weapons) by her extremely powerful Reaper meister.
Several in A Certain Magical Index and its spinoff. From the Railgun anime, we have Telestina. From the novels, we have Anyeze, who goes from ditzy, innocent fanservice girl to psychotic religious freak proclaiming Catholics believe that marriage with 'outsiders' is bestiality in the course of a single scene. Which is offscreen.
The 4th level 5 esper "meltdowner", while usually cold and calculating, once you kind of annoy her she kind of... melts down: her speech turns horribly vulgar and starts tearing apart her opponents with sadistic delight capitalizing on her massively powerful ability to basically blast holes through anything and surprising physical strength.
Czeslaw Meyer from Baccano — you know, his whole reason to come to NYC...
Euphemia, after she gets accidentally Geassed into becoming a mass murderer. "Those of you who call yourself Japanese, I have a favor to ask. Could you all die, please?" "I was hoping you'd all just commit suicide, but you can't, can you? O.K. So, soldiers, please kill the Japanese. Kill them all." Then she proceeds to grabs a submachine gun and shoot everyone.
Diva, the Big Bad of Blood+ often acts like an amusingly spacey example of The Ophelia, but she's pretty horribly evil.
A lot of child Contractors in Darker Than Black come across this way, especially the female ones, who channel a similar idea to Girl with Psycho Weapon. Mai in the first season of the anime, and both Azuza and Bai (in a flashback) in the interequel are shown killing or maiming people horrifically while having a Moe Stare expression on their faces. Maki might also count along with being a yandere. He sometimes comes off as an Adorably Precocious Child, but is completely psychotic and has an obsession with his leader, Amber.
Black Butler has Alois Trancy. Throughout the first episode of the second season, he flips from sweet and innocent to Ax Crazy so many times, it could make someone dizzy. As of his Character Development, he move into Yandere for Claude. That one backfired HORRIBLY.
Light Yagami of Death Note... utterly◊ adorablemass murderer, who was a manipulative faker years before he became actually evil. Though as L observes after Souichirou's heart attack, a lot of Light's acts are cheesy beyond belief.
Also Misa Amane is both Cute and Psycho and a Yandere.
Beyond Birthday from the Another Note spinoff novel. Take L's more Moe qualities and then add crazy-evilness into the mix.
Mairu and Kururi from Durarara. Since they're Izaya's little sisters, it was obvious. Other examples include Erika, Walker and Aoba.
Kumagawa of Medaka Box combines this with Stepford Smiler. His cheerful expression doesn't change no matter what he does; whether it's eating taiyaki (in the creepiest way possible), impaling people with giant screws, or blinding them.
The gun-runner manga Jormungand has no shortage of these:
First is Mildo, a twisted rival female mercenary introduced in Volume 1, who is seemingly obsessed with defeating H&C Logistics Incorporated's resident Knife Nut in hand-to-hand combat.
Mildo: Valmet, you're all I ever think about. How you live. How you fight. How bout we beat the living shit out of each other, then I kill you. Sounds fun, eh? Kinda poetic.
Tsubaki Kasugano, the Sixth Diary Holder from Mirai Nikki. She seems at first to be more of mix of Strange Girl, Yamato Nadeshiko and Bitch in Sheep's Clothing when dealing with Yukki and Yuno, but then we see that her backstory of orphanhood and sexual slavery drove her completely insane.
Fifth Diary Reisuke Houjou is a cross between Cute and Psycho and Tyke Bomb.
And, of course, Yuno herself! Typical of the postergirl for Yandere everywhere. She always has this adorable little smile on her face when she attacks people with axes.
Mikael from Tenshi Ni Narumon is usually a cute, easily agitated boy, but when under great stress and when things don't go they way he wants them, can turn into a really scary and obsessive psycho. It might caused by a side effect of his halo falling off.
The same can be said about Silky - very moe and doll-like but tends to smash everything apart and set on fire when enraged. Only she was a scary psycho for most of the time during the series and again it can be argued that it was mostly a side effect of her halo falling off.
Mama can turn into a very crazy witch when under too much stress.
Yukino-sensei in Hohzuki Island, who at one point sneaks around the school in the dark, naked, with a wooden sword, ready to beat one of the children to death. Later turns out to be a subversion—yes, she's snapped and is fucking creepy, but it was because she thought one of the children was Cute and Psycho themselves, due to her best friend and fellow teacher turning up murdered. The kids acting very suspiciously didn't help matters. By the end of the manga, she realizes everything she thought was wrong, clears up some misconceptions about her, and then pulls a Big Damn Heroes on the realmurderous teacher.
The Claw in Gun X Sword appears to be just a sweet old man.
Henri Claytor from Future GPX Cyber Formula, who appears to be a cute and cheerful rookie racer in ZERO, actually has a homicidal obsession with making Hayato pay for ruining his life. This is due to his father ignoring and abandoning him after Hayato wins the championship instead of him. He somewhat gets better after the end of ZERO.
If there are types of criminals that Shinichi Kudo from Detective Conan absolutely loathes, it's either this and/or Yanderes. And there's no shortage of those in such a Long Runner of a series.
Apparently, Dead Master in Black★Rock Shooter: Innocent Soul, as right before devouring a soul she calls the (naked) Rock tasty, ending it with hearts.
Fear from Cube X Cursed X Curious can become quite creepy when she decides to go fully psycho. It doesn't help that she IS a weapon able to become all kinds of torture devices.
Chaos from Heaven's Lost Property is what happens when you mix Ax Crazy with Love Freak while avoiding Yandere. What you get is the creepiest Moe character in the series who wants to share her "love" with everyone.
Terra I from the 1980's "Judas Contract" arc of The New Teen Titans. Seems like a sweet young girl; is actually a vicious and ruthless mercenary with distinct yandere traits — she's not only sleeping with the much older mercenary/assassin who hired her, she ends up killing herself (while trying to kill him) when the Titans make insinuations he's been unfaithful to her.
Fan Fiction
In The TruthAbout Bleach, murderous fugitive on the run Rukia qualifies, as well as Orihime, cleverly disguised as an Alpha Bitch until she brutally murders two soul reaper junkies over their stash of mod soul pills.
HetaliaDark Fic has an penchant for making the nations into this; the biggest victims (aside of canon Cute Psycho Russia) are Japan, Canada, Romania, Spain, America, and occasionally China and North Italy.
Agent Diamond, from Akatsuki Kitten Phoenix Corporation Overhaul, seems to swing between this and The Fake Cutie. She is generally sweet and happy a lot of the time... she just has an extremely frightening tendency to do things like kill people adorably. There may be a Freudian Excuse behind that, as well as basic nature: she "used to be" Renesmee Carlie Cullen and has only grown less human in mind since then.
Another My Little Pony: Friendship Is Magic example, in Bioshock-crossover Vision, Siren Song, who has come off as a Jerk Ass thus far, seems to be at it again when she doesn't have the slightest iota of trust for a kindhearted vet who goes out of her way to heal her. Turns out she was right, the second Siren rubs her the wrong way, the vet pulls out razor and starts raving.
Ganmed from Hivefled, who is "surprisingly CHEERFUL for a devout member of a faith which focuses so heavily on DEATH," has shades of this. He's an upbeat, kind of dorky late-adolescent troll who writes Self-Insert Fic and commits troll sacrifice without any qualms. However, much of the "psycho" is Deliberate Values Dissonance, as troll culture in general is pretty violent.
Film
The main character of the film Little Sweetheart, Thelma. She's a cute little 9-year-old girl (admittedly, with an odd resemblance to a certain The Good Son star, though that came second) who is skilled at blackmail and stalking, and willing to murder her only friend because she was going to ruin her plans.
Norman Bates in Psycho. He's a pleasant, polite fellow until you mention his mother.
Peyton Mott (played by a frighteningly believable Rebecca De Mornay) from The Hand That Rocks The Cradle. After her obstetrician husband kills himself to avoid facing several accusations of rape and she loses the baby she was carrying, Peyton "Flanders" sets out in an Evil Plan to ruin the life of the first victim who denounced him, Claire (Annabella Sciorra).
Please think long and hard before taking the polite and beautiful , seemingly nine year old Russian Orphan Esther home with you... It'snot worth it.
Becky from Drop Dead Gorgeous is a pretty, smiling beauty queen... until someone gets in her way. Then out comes her shotgun.
The Maids is about a couple of sisters serving as maids. Their mistress loves how sweet they are and how well they serve her. Poor madame just doesn't know what they do with her clothes when she's absent. Mistress is a kind of yandere herself, just not as wicked as her maids.
The hook of the Torture Porn film Neighbor is that the killer ("The Girl◊)" is an extremely ditzy, hot, and sadistic bitch.
Annie Wilkes, as played by Kathy Bates in the film version of Misery. Look below...
Eve from Alpha and Omega. She is normally a loving, smiling mother... but she has a preference to death threats that are funny only because of how she says them... unless you threaten her babies...
Sarah Jane McKinney in Miss Nobody is one of the sweetest people you could possibly meet... and she thinks little of murdering people to get ahead at work.
Lucindia◊ from Hell's Highway. Being cute makes it pretty easy to be picked up by people, who can then be brutally murdered For the Evulz.
Literature
As mentioned, Annie Wilkes from the Stephen King novel Misery is an archetypical example. A qualified nurse, she rescues the main character and her zealously favorite author, Paul Sheldon from a car crash in the middle of a blizzard and cares for him. Unfortunately, Paul later finds out that she is a psychopathic, schizophrenic Serial Killer who has no qualms against chopping off his foot with an axe and cauterizing the wound with a blowtorch if he acts up.
"Now I'll know. If anything happens to these dogs. If they run off, or limp, or anything - I'll kill you, your family, burn your house down, and sow salt. I have nothing to live for but these dogs...But if anything gets done to these dogs I'll decide it was you and I'll sacrifice my life and freedom to destroy you and everyone you love."
Isaac Asimov and Robert Silverberg's Daybreak recounted one particularly disturbing scene in which an important character is killed by a 5-year old lunatic girl.
Sharon Falconer from the novel Elmer Gantry and its adaptations. She cultivates the outward appearance of a successful, articulate preacher, but Elmer discovers that she is deeply disturbed. In private, she toggles different personalities, including a petulant child-like persona and a pseudo-pagan priestess.
Cassidy from Veronica Mars. Think he's the sweet, nerdy, helpless guy? Ha. More like rapist and mass-murderer. Have fun!
Elle Bishop in Heroes, a cute, childish, and sociopathic woman who delights in killing, hurting, and controlling people.
The Trinity Killer from Dexter. It's also an inversion; we see the dark side first. It's the "nice guy and family man" side that comes as a complete shock.
All That's "Ask Ashley" featured a sickeningly sweet little girl with an insanely unstable temper answer moronic viewer's mail. Hilarity Ensues.
Gem and Gemma of Power Rangers RPM are a mild case. Mentally, they're about kindergarten age, and their favorite things in the whole wide world are explosions. Especially when it's the evil Killer Robots going boom. The reason they're only a "mild" case is that they don't really want anyone hurt (though there was one time they had to be reminded that there were civilians enslaved in a factory they planned to "vaporize from sub-orbit").
The third series of The Thick of It introduces us to Steve Fleming, MP, who is a spectacularly unstable version of this trope; that cheerful grin, the slightly creepy compliments and the "call me Uncle Steve" attitude you see when you first meet him? Get him even slightly agitated and his Ax Crazy side will come to the fore. The scary part comes when he desperately tries to suppress his insanity, swinging from Stepford Smiler to Unstoppable Rage and back again so violently you wonder why he doesn't give himself whiplash.
Hooch from Scrubs. He's normally friendly and even charismatic, but pissing him off triggers his Ax Crazy tendencies. This leads to J.D. and Turk frequently playing pranks on him to trigger his psychotic side, at one point even eating popcorn while Hooch is throwing death threats around. It becomes clearer with every appearance that the catchphrase "Hooch is crazy" thrown around by other characters is not exaggeration. After another of J.D. and Turk's pranks, he takes four people hostage and is subsequently fired from the hospital. In the very last episode of season 8, we see him in a strait jacket, proclaiming "Hooch is crazy."
Rachel Berry definitely is a high-intensity-low-stability kind of girl on Glee, with her Serious Business approach to... well... pretty much everything. It's absolutely adorable when it's not just infuriatingly annoying, especially to her fellow Glee Clubbers who just seem visibly tired of her antics at times.
There's also Quinn, who can be incredibly sweet and endearing... When she's not screaming at someone or going to absolutely insane extremes just to feel good about herself again.
Bernadette from The Big Bang Theory is a mild version of this trope, usually sweet, demure, innocent, even a bit of a woman child at times, but she has a very short fuse and is extremely impatient and will go off on someone over seemingly small things and is quick to give Howard ultimatums if he has a dilemma that involves her. She also almost never stops smiling. And it's always the exact same toothy smile, even when she casually mentions the potentially weaponizeable biological specimens she's working with, or threatening someone with grievous bodily harm.
Another mild example is Annie from Community: she is generally sweet and well-meaning, but minor things like a C grade or a lost pen have been known to make her crazy and/or violent for a while.
AJ Lee of Chick Busters fame is one of the best example. Lithe, really pretty, she was with CM Punk (because he digs crazy chicks) before trying to seduce Kane, of all people, and kissing Daniel Bryan on the top rope before putting him through a table.
The Maenads. Dionysus's followers who were once innocent, kind-hearted women until too much magic alcohol caused them to do horrific acts by tearing apart animals, children and sometimes men with their bare hands. They also killed poor Orpheus.
Loki. Handsome, clever, funny, well-liked—but sadly, not at all mentally stable...
Similar to the Maenads mentioned above, there were presumably some berserkers who were somewhat endearing, or at least relatively non-threatening, when they weren't going berserk.
Tabletop Games
Forgotten RealmssourcebookShining South got a sample NPC like this. A smiling half-drow (and both halves do work in her favour) cutie with Girlish Pigtails. In a tasteful armor, with short sword and composite bow. According to her stats, a Talos-worshipping Neutral Evil ranger / shadow marauder. As in, a Dambrath noble pillaging neighbour countries more for the fun of it than any economical benefits, and trained to not be stopped by boring things like fortifications for longer than an eyeblink.
Video Games
Venus from Metal Gear Acid 2 is an extremely beautiful Femme Fatale, and seems flirtatious, sexually confident, and yet somewhat vulnerable at first. She also kills a frightening amount of people in cold blood and seems completely incapable of actually feeling normal emotion despite her seductive, romantic play-acting. It's easily her sexuality which is her most screwed-up trait - during the game, she gets aroused at least once by dowsing (in a Does This Remind You of Anything? comedy scene), shuriken, murder, a corrupt genocide incident, a Mad Scientist experimenting on little children, her partner killing people, and her partner nearly getting blown up. Her partner, naturally, is creeped out horribly by her.
There are a few male examples: Matt Engarde in Justice for All, Kristoph Gavin in Apollo Justice and Souta Sarushiro in Ace Attorney Investigations 2.
Selphie Tilmitt in Final Fantasy VIII. Sure, she's a cute and bubbly sixteen-year-old girl, but don't forget that she is a highly-trained Super Soldier mercenary and a particularly unstable and violence-loving one at that. Examples include suggesting to skinning a moomba to create a disguise, cheerfully suggesting to blowing up a train with a rocket launcher, and destroying an entire missile base with a smile after they launched missiles at her old military academy. Even the other protagonists, also from said elite mercenary unit, are disturbed by some of the stuff she comes out with.
Touhou has Flandre Scarlet who, after spending almost five hundred years in the basement of the Scarlet Mansion to protect people from her immense powers , has the mind of a child and a poor understanding of restraint, only wanting to play but with a tendency to "break" those she plays with. She isn't exactly murderous, but she's unstable enough that Marisa - who has fought some of the most powerful denizens of Gensoukyou, which includes a few resident Physical Gods - actively avoids her whenever she goes to the Scarlet Mansion to steal books.
There's also the floweryoukai Yuuka who might or might not be the most homicidally insane being in the whole huge cast. No-one really knows if her comment about genocide just being a game was her honest opinion about it or if she just said it to see what kind of reactionthat statement would provoke but, frankly speaking, it doesn't matter: People avoid her whenever possible. Not only because of her unsettling statements but also because she has enough power to back up her threats about turning people into Pink Mist. The fact that she tends to say all of these things in a polite voice and with a gentle, cheerful smile probably doesn't make her any less unsettling to those she speaks to, either. There's also the fact that she is YoukaiMoe~.
Teemo in League of Legends, a Mooglerip-off who couldn't care less about all the people he kills, but is slowly cracking from prolonged isolation from others of his kind.
Lovrina of Pokémon XD: Gale of Darkness fits the bill perfectly. A cute, ditzy Valley Girl with a penchant for 'beautiful' Pokémon... who is also the Evil Genius of a criminal syndicate responsible for the routine Mind Rape of Pokémon in order to transform them into soulless killing machines. She turns very nasty when pushed, and one of her subordinates describes her as "nothing short of terrifying".
Twins Mireille and Mischka in Valkyrie Profile: Covenant of the Plume, the former being female and the latter male. They kill purely for fun and hold a total lack of compassion for those they fight against, and ultimately kill - even when it is not necessary, such as butchering a soldier who surrenders. Neither of them care to even hear the plight of rebel leader Natalie, who they kill. She is implied to be their mother, unbeknownst to them. This disgusts even Wylfred, a character who sacrifices his own friends and childhood companions for the sake of revenge.
Blackbird from Strife. She actually gets excited when you find new weapons and starts talking about their killing potential. Not to mention that this seems to be her preferred method of dealing with most problems that come up.
In Blaze Union, we get Eimi, also known as Garlot/Gulcasa's sister Emilia. Nothing could be more adorable than her in the whole game... But if you go for the B route and field her too many times in battle, her love for her Brother Captain and her other friends, together with her desire to be useful to them, ends up turning into what drives her over the edge. As a result, Brongaa's blood manifests at full power in her, and she becomes a monstrosity that will remind many Deptheavenites of Brongaa Possessed Gulcasa. At this point, she is bent on nothing but destroying anything that stands in her way, and Garlot is forced to put her to the blade, both to put her out of Brongaa's torment and to assure his survival and that of everyone else.
Professor Layton and the Unwound Future has Klaus/Clive, sweet and polite in his mannerisms, and especially helpful to those in need. At least, until he reveals his plan to completely level London in order to rid it of the corruption that covered up the explosion that killed his parents.
The same game also features Puzzlette, who acts all cutesy and bubbly and sweet even while she's trying to murder sapient insects. Then she just casually tosses her flyswatter away and starts chattering to you about puzzles like nothing's out of the ordinary.
Egbert from Suikoden V. He really is a very polite, peaceful, and friendly fellow. Until something gets his blood boiling, and then he flips out and begins ranting. Usually triggered by the mere thought or mention of the Godwin family.
Ripper Roo from the Crash Bandicoot series, while at first completely insane, becomes like this after the epilogue of the first game, where he is stated to have undergone intense therapy and higher education.
Iris Sepperin from Rosenkreuzstilette is generally a prim and proper little girl over the course of the first game's story. Then, just before the final battle, she starts laughing madly and quickly shifts from her affable attitude to motormouthed insanity and arrogance during her Villainous Breakdown.
Tiny Tina from Borderlands 2 decorates her house with bunnies and is fond of tea parties. In which she uses electricity to torture a bandit to death. Said bunnies are also filled with explosives, the use and making of which Tina is very proficient in. She also talks like a pimp and describes the aforementioned bunnies very... strangely. Yet, she's still extremely huggable.
Harley Quinn has always been this, but in Batman Arkham City it goes to new heights. Her goons think she is worse than The Joker.
Noire in Fire Emblem: Awakening. She comes off as a delicate and soft-spoken young lady who has a timid demeanor due to being used as a guinea pig for her mother's curses. However, also BECAUSE of that, she has formed a split personality that surfaces whenever she gets fed up with the antics of her comrades or during the heat of battle. This side of her is also a bit of a ham.
Noire: BLOOD AND THUNDER!!!!
Tharja also qualifies, as she's a beautiful, but rather creepy Yandere who enjoys practicing curses. Unsurprisingly, she happens to be Noire's aforementioned mother.
Higurashi No Naku Koro Ni features some of the more well-known ones, such as Rena Ryugu up there in the page image. A lot of the times the characters are delusional, so that either makes them Cute and Psycho or makes them appear Cute and Psycho. It's implied that all the central characters except Mion, Rena included, contracted Hinamizawa syndrome during many other iterations of the Groundhog Day Loop each; Rena would be a straight example in every one where she did. We're just only shown one.
Those same clips give you a hint of her mother Rosa's version of this: she can swap between loving and motherly and abusive monster at the drop of a hat. This being the result of her being severely bipolar combined with possibly having PTSD as the result of a harsh childhood and certain incidents shown in the 3rd novel.
Kozue in Chaos Head. For a Cute Mute, she sure seems awfully indifferent to the idea of killing people. Well, killing jerks. Hell, she got her sword in the first place so she could kill.
Played for laughs with Riko in A Profile. As Masayuki's stepmother, she's very easily influenced, strong willed and tends to have thought processes at right angles to any normal person. Thus, she is prone to brainwashing her children and any number of other odd things.
Shiraki Aeka from Yume Miru Kusuri is a horribly, horribly justified example.
Ilya in Fate/stay night, which is rather baffling to Shirou. Eventually he decides it's because she has no sense of right and wrong, and instead acts on any desire she has, no matter what it is.
Kazuaki Nanaki (aka Hitori Uzune) in Hatoful Boyfriend. He's a sleepy, gentle teacher who just happens to be so traumatized by the death of his brother Nageki that he's constantly haunted by a shadowy manifestation of his guilt that he believes to be Nageki's ghost hating and blaming him for his own death that drives him to do some truly creepy things near the end of the Bad Boys Love route when he begins talking about how "Nageki's calling for me" and how he's going to "take Nageki home with me" as he attempts to slice Ryouta open with a knife so that he can remove Nageki's organs that were implanted in him, descending into a Madness Mantra as he does so:
The second game reveals that he got his current identity from a deeply depressed friend he encouraged into committing suicide, claiming they would do it together. When the friend changed his mind after taking pills, Hitori said it was too late and let him die. In the 'present' another character tries to help him and is almost strangled, and correctly concludes the following.
"He looks pretty gentle on the outside, but I think he's pretty messed up inside."
Buwaro from Slightly Damned is a demon, but he loves everything and everyone. He'll invite you to play catch, or explore, or just give you a great big hug. But if you value your life, do not remove his star pendant! While he goes berserk without it, it is not a Berserk Button, but a medical condition which the pendant is magically treating.
Errant Story: Concussion is a cute little fairy nurse eager to help you. Her name is not an oxymoron, it is an euphemism.
Bangladesh DuPree of Girl Genius. Smokin' hot? Check. Buttfuck-insane murderous pirate queen farming out her sadistic tendencies to an employer inclined to use them constructively? Checkaroonie. Perky and nonchalant about the whole business? Check, check, a thousand times check.
Zoophobia has Cameron, a go-getting, curious young woman, who happens to be psychotically neurotic when in uncomfortable situations, or when around animals.
Three examples come to mind on That Guy with the Glasses; Ask That Guy, with his poshness and suaveness hiding a monster, Bennett The Sage, who looks sweet and cute but can terrify Ask That Guy, and The Nostalgia Chick, who really is girly and generally decent, but has a mess of issues and sociopathic qualities underneath.
Pink and Blue from Charlie The Unicorn. Cute little innocent sickeningly optimistic and 'imaginative' unicorns, who pull elaborate traps for Organ Theft.
The Laughing Clown from Dark Dream Chronicle types in pink and generally acts like a little kid. She's also dangerously psychotic, once biting someone's tongue out for "shiggles". Even the othermembers of the cast think she's completely insane.
Survival of the Fittest has a fair amount of these sort of characters, mostly of the Beware the Nice Ones variety. A textbook example of this trope, however, would be Mariavel Varella of v2. Due to abuse at the hands of her father, she can go from a cheerful teenage girl to a hardened Delinquent who apparently once beat a man to death with a baseball batbefore being abducted. And when she gets to the island... well, she just gets even more crazy.
Jade Sinclair (codename Generator) of the Whateley Universe. She's cute and sweet and cheerful. She looks like she's ten or eleven. She 'invents' wacky devises. She looks like the harmless member of Team Kimba. Right up until she handles an attacker by stapling him to a tree with railroad spikes through his arms and body.
Ren from The Ren & Stimpy Show varies in cuteness, due to the often grotesque art style of the show, but he IS a rather cute little chihuaua with big eyes who's also prone to psychotic breakdowns and acts of uncontrollable violence.
Terra from Teen Titans is somewhat like this. She seems friendly, funny, and kind, although her Dark and Troubled Past has saddled her with destructive tendencies that can, and if helped along by Slade, will, manifest as homicidal psycopathy. Her switch from a cool gal-pal to Slade's Apocalypse Maiden was astoundingly dark, given the show's track record at the time.
Jodi from Titan Maximum is showing signs of being this.
Flame Princess has a Hair-Trigger Temper and she's destructively powerful, capable of setting kingdoms (and at one point the entire planet) ablaze and she's evil; she could get mad at any time. But when she's not mad, she's really cute.
Pinkie Pie's Genki Girl nature survives most things, and if it doesn't most of the time it means she only becomes depressed. But you should still never break a Pinkie Promise.
Princess Luna is this in Physical Goddess form. She is the very definition of Adorkable and has the absolutecutest facial expressions in a show that seems to run on cute facial expressions. Yet the first time we meet her she is an Omnicidal Maniac who is trying to bring forth The Night That Never Ends and is consumed by envy of her sister's greater popularity. Even after she is cleansed of all evil she retains a Hair-Trigger Temper which expresses itself through her fondness for summoning lightning in the sky to add emphasis to her sentences.
Sonia also counts. She looks like a sweet little girl who doesn't belong in a world of kids fighting adults, but then you tell her you're going to shove her into a dark place. Prepare to get your ass kicked.
Metalocalypse has Toki Wartooth, Dethklok's adorably immature rhythm guitarist, has what could be called an unconventional mental state. Sometimes, this manifests as pastel-colored acid-trip musical numbers. Unfortunately, he also has substantial anger management issues, particularly when the show's been putting him through a round of Break the Cutie.
Kaeloo. While her Jekyll-and-Hyde-esque split personalities physically differ (cute, innocent frog and violent tower of toad muscle respectively), they overlap so heavily in the later episodes that some scenes give off the impression that at times Hyde Plays Jekyll.
Played for laughs with Stewie Griffin, a one-year-old homicidal mad genius.
Joan is shown to be very sweet and she falls in love with Quagmire. The two were set to be married, but when Quagmire went to express his doubts, Joan does a complete 180 in her mannerisms and threatens to kill herself and Quagmire.
Izzy from Total Drama is this in spades. In Revenge of the Island, Zoey is this andBeware the Nice Ones personified after she's pushed too far.
