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, though while there is frequent overlap between these tropes being one doesn't necessarily mean a character qualifies for another. 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. The Ophelia is someone whose psychosis is part of the cute picture, rather than a contrasted to it.
In some anime fandoms this character is referred to as a yangire, an informal fanspeak term which is a portmanteau of yandere and kireru (a word meaning "to snap or lose one's temper"). It's also used to refer to Ax-Crazy versions of The Fake Cutie. Not to be confused with fangire, which is a species of monster vampire.
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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.
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."
Wrath from Fullmetal Alchemist 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.
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.
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.
Deadman Wonderland has 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 is a mixture of Cute And Psycho and Yandere. He cracked for non-romantic reasons (the Bloody Sunday), has mood swings due to his childlike temperamentand manipulative nature, but his unstable side comes out especially strongly in regards to those he wants to keep close.
Latvia has the potential to become one, 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.
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.
Czeslaw Meyer from Baccano! — you know, his whole reason to come to NYC...
Code Geass has 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.
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.
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.
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. Valmet: No... not at all... you psycho!
Tsubaki Kasugano, the Sixth Diary Holder seems at first to be more of mix of 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.
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 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×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.
Fear from Cube×Cursed×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.
Yagami Yuu from Night Shift Nurses. When she's not being a good nurse who makes her patients comfortable, she's a psychopathic, murderous nymphomaniac.
Nui Harime from Kill la Kill. She never lets up her playful and carefree nature, wears frilly dresses, and loves breaking the fourth wall. She also gleefully recites how she was responsible for killing Ryuko's father. She didn't even let up her childishness while she sunk the Rending Scissors into him in a flashback.
Kanon Nakagawa from The World God Only Knows. She's the epitome of the sweet, adorable idol... as long as you don't get on the wrong side of her stun guns, as Keima did. Kanon and Keima's very first interaction was the latter telling the former that he doesn't know who she is. Her reaction is to taze him repeatedly.
Rio Miyaichi from Ana Satsujin. She's a cute, socially awkward college student, but also a psychotic serial murderer who favors slashing her victims necks with a box-cutter.
Akame ga Kiru! gives us Seryu Ubiquitous, the "Crazy Lunatic". Regularly, she's sweet around those who she doesn't consider evil. Around those that she does, of course... her personality switches over to bat-shit insane. As a matter of fact, she comes complete with a twisted conception of right and wrong.
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.
The Truth About Bleach has murderous fugitive on the run Rukia, 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.
A good part of the Axis Powers Hetalia concept 2P!Talia relays on all or some of the Alternate Coloring Characters fitting in this trope, even when there is next to none canon info on them aside of their looks.
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. Much of the "psycho" is Deliberate Values Dissonance, as troll culture in general is pretty violent. However, a few of the more stable cast members do suggest that a moirail would do him good (the implication being that he needs a stabilizing influence).
Harry/Tristan in The Jaded Eyes Series. It's lampshaded when Tristan, is in his next incarnation, is seen and pitied by one of the slaves who thinks that that poor child being held captive by the Dark Lord... until she realizes that the child isn't being held captive and is in fact far worse than the Dark Lord.
"You are just too damned cute, you know?" Kira shot him a glare. "I'm not cute," Light snapped petulantly.
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.
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...
Speaking of Kathy Bates, she plays a seemingly friendly Squirrel Lady in Rat Race. Two women in a car stop where she (and her squirrels) are on the side of the road. She tries several times to get them to buy a squirrel, but they only want directions. She gives them very specific directions before sending them off with a paper bag marked "I <3 SQUIRRELS". The women follow the directions that send them down a very steep incline. As they plummet, they pass several signs with one word written on each: "YOU... SHOULD... HAVE... BOUGHT... A... SQUIRREL". They end up in a ravine atop a large pile of vehicles...and a skeleton holding an I <3 SQUIRRELS bag falls onto the windshield.
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.
Belle Blake, from Malavita, is a cute blonde teenager who just happens to be proficient with weapons and beats the crap out of a sleazy boy with just a tennis racket. Justified as she's a Mafia daughter in hiding with her family.
The little girl from The Bad Seed. So pretty. So polite. So well-behaved. She's also a sociopathic serial killer.
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."
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.
In the novel and movie "The Bad Seed", Rhoda Penmark is an angelic little girl who loves her mother "Thissss much" except when she's drowning a boy who won the spelling medal she wanted, setting a dim-witted man on fire because he seemed to know too much, pushing an old woman down a flight of stairs for her snow-globe
Alina Galenko from the web novel Animas Conquest is a charming, pigtail-sporting neo-hippie of sorts who is absolutely not above kidnapping and terrorizing the faculty of Salva Academy.
The Poem from The Dollmaker is a tiny and beautiful woman. She's also a horrifying sociopath.
Live Action TV
Angel: Fred's tendency to be this is never completely gone - just dormant.
In Game of Thrones, a cruel process of Break the Cutie has brought this out in young Arya Stark, whose "evening prayers" now consist of a list of enemies she intends to kill. It's illustrated at its best near the end of season three, when she calmly informs Sandor Clegane that she will stick a blade through his eye and out the back of his skull some day. The next episode, she hammers it home by catching a Frey soldier off guard with feigned cuteness and vulnerability, then repeatedly and visciously stabbing him, all without showing the slightest remorse. Keep in mind that she's barely more than a child.
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 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, 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.
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.
Buffy the Vampire Slayer: Drusilla Keeble is soft-spoken, loves dolls and has a child-like innocence and affection for her "family" which can on ocassion come across as vulnerable and sweet. She's also a omnicidal, certifiably insane vampiress who has on ocassion been known to slit throats with her own nails. She was lead to believe she was evil by Angelus who stalked, tortured and assaulted her for years until she finally turned insane and is turned into a vampire so she can suffer for eternity, so by this point the "psycho" part of this trope is quite understandable.
In her youth, before her Heel-Face Turn, River Song was raised as a Tyke Bomb by an Ancient Conspiracy who intended her to murder the Doctor, but she was still as sexy and quirky as the good River we know and love.
Mel from Flight of the Conchords. She does things like bake the band the cookies and has a not-so-secret crush on them that leads to some awkward moments between them. This description, although while accurate, would give you the impression that she was an AdorkableDogged Nice Girl. But she's not, she's actually an insane Stalker with a Crush. And those awkward moments are the result of her creepiness and the fact that she sometimes coyly hits on them in front of husband.
On the Soap OperaThe City (a retooling of Loving), the cast was being stalked by a Serial Killer. Much to the audience's shock, it was soon revealed to be petite, perky blonde Molly Malone.
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.
While bordering on and sometimes crossing over into Miss Fanservice in her initial WCW run, this trope was the case in TNA. On TNA Xplosion, Don West described Daffney as "Just crazy enough to attract you and just crazy enough to make you want to get the hell away." When forced into a strip tease after Impact went to Monday nights she proceeded to give perhaps the most not sexy undressing a genuinely attractive person ever gave on Spike TV, as her eyes screamed "Die". She also got herself cutup more than all the other knockouts, perhaps barring Roxxi, usually when her own designs to maim people backfired.
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.
Venus from Metal Gear Ac!d 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.
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.
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. To be fair, this is a First-Person Shooter, so powerful weapons usually are a great way to deal 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 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.
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.
For all that Selphie of Final Fantasy VIII is a sweet little Genki Girl, she's also far and away the most violently pragmatic of the player characters. Whenever she's involved in the planning phase of a mission, she always seems to suggest using explosives. When planning how to escape D-District, while a Moomba is in the cell with everyone, she casually suggests skinning it and wearing it as a disguise. It's hard to tell if she was actually joking or not.
Noire 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 in the Bad Future she is from. 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!!!!
Henry is unfailingly cheerful, but amost every single line coming out of his mouth reveals his very screwed-up psyche that comes from a horrifyingDark and Troubled Past.
Jenny Jaros of the Saints Row: The Third expansion Gangsters In Space. Hoo boy. She's taller than Female!Boss, plays a Purple Skinned Space Babe, incredibly fit, and clearly works out judging by her figure. She starts out as a true Nice Girl in awe of working with the leader of the Saints in a film, and the director punts her like some Jerk AssTake That combination of Hideaki Anno, Christian Bale and Joss Whedon. Boss; who can easily qualify for this trope themselves, pets, strokes, nurses, caresses and generally takes her under her\his wing in the face of all this abuse, and when the director goes one step too far and tries to replace Jenny she reacts by gunning down the extras with prop firearms, then beating them to death if Boss stays around. When the Director reveals he is actively trying to kill them in the movie Jenny snaps, and in a scene that frightens even the Anti-HeroBadass In ChargeDevil in Plain Sight Boss runs down the director in a spaceship, killing him. Apparently Jenny has anger management issues but hot damn was it not deserving.
Professor Genki as well, being a big, cuddly-looking pink cat with comical Nerd Glasses who yells in enthusiastic broken English... and loves him some Murder Time.
Phantasy Star Online 2 has Lisa, who's a Cast and an instructor for rangers. She's always smiling and full of enthusiasm, but her conversations typically revolve around her love of filling things with lead and/or blowing them up.
There are many cutesy and lovable demons in Shin Megami Tensei, like Mascot Mook Jack Frost, Pixies (most Fairies), or even recurring demon Alice. None of them will hesitate to rip you to pieces if you aren't quick to deal with them with either firepower or some damn convincing gab.
Mogeko Castle has the Mogekos, little yellow cat-like creatures with curly tails who love prosciutto... and also high school girls. In very disturbing fashion. They love playing with high school girls, and don't particularly care about consent.
Also from Mogeko games, Rawberry from The Gray Garden, a cute little pink-haired demon who will murder and devour anything in her way when she's not sleeping. Including her teammates.
Higurashi: When They Cry 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. Well, there is another example of her going Cute and Psycho in the PS2 arcs.
Her mother Rosa 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.
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) 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, descending into a Madness Mantra as he does so. 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."
Irina from Rasputin Catamite looks like a cute porcelain doll with pigtails, and speaks in third person, but seems to enjoy hacking people to death with an ax.
Sophie from The Overture is the most vile example of this. She uses her child-like appearance and sweet singing voice in order to lure children into her grasp. She then proceeds to torture them in order to hear their screams.
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.
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.
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.
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, and burns a message into his flesh.
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.
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.
Sonia 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.
The title character of 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.
Family Guy: 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.