Akira himself, as the most powerful of all the Espers who is also a Cute MuteEmpty Shell, completely dominated by Tetsuo.
Luna Koizumi from the Alice Academy manga is an extremely Creepy Child, not only two-faced, but also being able to charm people into supporting her, while taking everything away from Mikan to make her life a living hell. She even tries to strangle the poor kid!
The Child. The Child only appears on nights of the full moon, and has no name nor does he speak. The Child seems to have strange powers of controlling demons just as his misshapen form did. He has an uncanny resemblance to Guts and Casca and is strangely attracted to both of them, but he mysteriously disappears just as he reappears.
Rosine, too. She's the youngest of the Apostles and their local Dark Magical Girl, extremely broken due to her horrible past, also insanely powerful as well as cute-looking. And she's obsessed with Jill, a pre-teen girl who once was her best friend...
Ciel's a cynical, jaded, and very driven 12-year-old (or 13, depending on how far you are in the series) who is a Chess Master and wise beyond his years. He has no qualms about getting involved in dangerous missions and will command his badass demon butler Sebastian to kill someone without a second thought. Then again, there's a reason why he is the way he is now.
Alois from season 2. He's Ciel on scary.
Alois's younger brother Luka. Being rather pleased that your Deal with the Devil killed a village is weird.
Yachiru as a toddler was drawn to blood to the extent that she was trying to touch Kenpachi's blood-soaked sword when she first met him. Now she's the lieutenant of the Blood Knight division. For all her cherubic appearance and cutesy ways, she's blood-thirsty, as excited by Kenpachi's fighting and killing as Kenpachi himself is, her reiatsu can terrify the messengers of captains, and her entire division is terrified of her. In hindsight, it was kind of obvious that she was the true spirit of Kenpachi's zanpakuto.
Gremmy Thourmeaux from the Vandenreich is this and more. He's a pre-teen who takes pleasure in how he can use his imagination to kill. His smiles are manic and insane and his eyes are vivid red, with no visible difference between sclera, pupil and iris.
Btooom! has Kira Kyousuke, a 14-year-old boy who likes to crush small animals and who raped three women to death.
Bungou Stray Dogs has Q, a 13-year-old Port Mafia member whose forced isolation seems to have stunted his development, making him come across as closer to 9. He carries a Creepy Doll everywhere and has the ability to produce hallucinations based on a person's worst traumas, which he takes great pleasure in using. As a child he killed dozens or more Port Mafia members and was considered so great a threat that he had to be sealed away, only to be released later when the Mafia was pulling out all the stops against the Guild.
Also from the Port Mafia is Elise, although technically she's not a child, she's an ability. Her first appearance has her scribbling a picture of Akutagawa with his head cut off while Mori and Higuchi discuss the possibility of a group Akutagawa angered retaliating against him.
Joshua Christopher from Chrono Crusade becomes this after Aion puts Chrono's horns on his head. Although most of the time we see him he's 14 and doesn't look quite like a child anymore, in his crazier moments he still acts like he thinks he's ten—which only helps the creepiness.
If in Claymore you see a little girl running around with little to no clothes on then you better run for your life, as she will most likely be an extremely powerful creature able to torn Humans and monsters to shreds in the blink of an eye. Claymore seems to really like this trope as it gives us three different examples of it.
Miata, a high ranking Claymore and traumatized Wild Child with Super Senses. As long as her motherly figure Clarice is around she acts like a very small child would, even wanting to be breastfed, which isn't normal for a kid of her age. Before Clarice came around, and every time Clarice is endangered however Miata displays a reallyaggressive behaviour toward anything.
Priscilla, the Big Bad of the series, at some point and for some reason mentally regresses to the state of a child. She's still an obscenely powerful monster though, enough to scare away Creatures of the Abyss like Riful. Even after regaining her memories she still uses, and fight, in this form.
V.V. looks like a cute boy with lustrous Rapunzel Hair but he's actually a man in his 60's, because he got his Code at an early age so his physical growth was stunted. He is the older twin brother of Lelouch's father, Charles vi Britannia, and he murdered Lelouch's mother, Marianne vi Britannia entirely out of jealousy when she started getting too close to Charles for his liking. He's head of the Geass Cult, a rather shady, creepy society in and of itself which produces tyke bombs like the ones described below); and he tries to kill Lelouch, Charles son and his nephew. He is the creepiest character in the series, by far.
Rolo Haliburton/Lamperouge, who starts off as a Tyke Bomb sent to spy on and eventually kill Lelouch, until Lelouch uses the feelings of brotherly love Rolo's built up over the past year to manipulate him with the full intent of killing him once his usefulness runs out. This proves to be effective because Rolo, as a raised-from-birth assassin, has never known any form of real kindness. Unfortunately, this backfires when Rolo becomes so obsessive and fixated on being the only person his "brother" cares about that he murders Shirley, one of the few people Lelouch was genuinely close to, and plans on killing Nunnally, who's Lelouch's whole reason for living. He does manage to redeem himself by the end though.
There's also the scene near the middle of R2 when the Black Knights are in the laboratory where the engineered Geass-children are. About four of the children, after Geassing a Black Knight to shoot another, look up at Rolo and shout, "Big Bro Rolo, is that you?" all smiling and whatnot. Then Rolo smiles back, and promptly shoots them.
Adorably Precocious Child Maki. He'd be a little creepy even if he was just a sociopath with the power to make stuff blow up, but he also has a massive, crazy stalker-crush on his boss, Amber. Such an insane crush, in fact, that he completely abandons the usual Contractor "emotionless" act when he finds the guy Amber's in love with, and tries to kill him even though if he'd succeeded, it would have ruined Amber's entire plan, resulting in a genocide.
Any child Contractor has these traits. They are disillusionedkilling machines, who lost their emotions and consciences, due to Gate's influence. Just like their adult counterparts.
2nd Grade Undertaker Daida "Punishment" Hibana, who knows way too much about various methods of torture having apparently learned from her mother, shown in flashback watching over her "suspension". Understandably, she is a bit delusional having to invent nonsensical punishments for herself after her mother apparently committed suicide.
There's also Minatsuki Takami when she was a child.
The Creepy Twins Ichi and Hajime, who at first seemed to be a single, creepy Forgery kid with a lollipop.
In Detective Conan, Edogawa Conan and Haibara Ai might not actually be7 year olds, but they certainly come off that way to everyone else. Conan is undisturbed by every gruesome murder they come across as he walks up to the body and looks for evidence. Haibara doesn't even bother acting like a little kid, foregoing that for her sarcasm and incredibly cynical worldview. And her occasional suicidal tendencies.
Besides, being 17 and 18 isn't that much less creepy when they're acting like that.
Sometimes Conan/Shinichi is just trying to be creepy. There's no other explanation. Like in a conversation between him and Takagi, right after Conan tells him exactly why they have other option but waiting to defuse a bomb in the same room as them until seconds before it explodes, in order to get a hint about where the next bomb is. It's even worse because Conan is the only one who can get to the bomb to defuse it. Because Takagi doesn't know that Conan is really a seventeen year old genius detective, this conversation follows:
Takagi: Tell me one more thing. W-Who on Earth are you?
Conan: Oh. Well if you want to know... Then I'll tell you. In the next world.
Mariko Kurama or Number 35; appearing like just another cute little girl until she rips you open with her 11 meter long, inhumanly strong 26 invisible arms which she views as a fun game thanks to how she was "raised" (if we can call THAT "raising"), and that her victims are her playthings. In the manga she has even more arms with an extended range as well.
There's also the cruel kids from the orphanage Lucy lived in when she was younger. They often harassed her for her cute little horns, emotionless nature and loner status, and also because they were miserable themselves. Also, after she started caring for a puppy, the others forced her to watch as they kicked the dog and beat it to death with a vase, just to try getting her to show something. Even creepier in that the whole time they did this they all sported huge slasher smiles and laughed at her. They got more than they bargained for: Lucy snapped and left no witnesses.
There's also a girl that pretended to be friends with Lucy and later sold her out to the mean boys by telling them about the dog, fully knowing that they'd use the dog to torment Lucy further. She even cries for Lucy upon finding out that the boys beat Lucy's dog to death and made her watch, but as she does this we see her smile and her fake crying turns into peals of laughter.
In episode 9 there's a scene where a young Lucy hallucinates and sees the mutilated dead bodies of the cruel kids come back to life and mock her. Afterwards she is seen talking with the voice of her DNA inside her mind, leading to her attempting to strangle Kouta. After finding out that Kouta lied to her about the gender of his cousin he was going to a festival with she goes on a rampage, murdering everyone in sight, including Kouta's father and sister, and tries to go after Yuka but Kouta tackles her and demands that she stops... Yeah, Lucy was this.
In the Fullmetal Alchemist manga and the Brotherhood series, Selim Bradley starts to show Creepy Child tendencies once he's revealed to be the homunculus Pride. He's an Eldritch Abomination.. Bonus points because that form is his true body, so he looks that way while cutting people in half with his eye-filled shadows and eating his own allies, sometimes all done while smiling. As The Dragon he comes as the most terrifying and just plain evil of the Homunculi bunch.
In Full Metal Panic!, Sousuke, as a result of his heartbreaking backstory. He goes from being a sweet, innocent kid to a child soldier who kills without batting an eye, has an extremely nihilistic view of things, and acts creepily unemotional. Especially noticeable here. In one of Gauron's flashbacks we see Sousuke as an 12 year old child soldier in Afghanistan, casually tossing bodies into a fire.
Kalinin: It's my belief that my nation... stripped that boy from his humanity.
Though, in Gauron's twisted world view, Sousuke was less the "Creepy Child" and more the saint of battle...
James, of Gregory Horror Show, appears to be a typical Hotel Hellion who only wants to play. However, his idea of "playing" involves putting people in danger (his grandfather nearly chokes to death while James uses his body as a swing in one episode, and he doesn't even seem to care), and on several occasions he deliberately puts the Guest in life-threatening/terrifying situations, chuckling happily all the while. When the hotel is set on fire at the end of the second series, James uses it as an excuse to play "firefighter," and in the next episode acts very cheerful as he talks about how "cool" it was for his skin to burn off.
In Guardian Fairy Michel, when Kim first meets him, Michel's supernatural calm is a little bit unnerving. He becomes more energetic later on, though.
The title girls from Gunslinger Girl appear to fairly normal children — at least to outsiders. During their missions they change into relentless killing machines, showing hardly any emotion whatsoever, contrasting heavily with their innocent looks. Especially the actions of Rico are often jarring, since she is prone to kill innocent bystanders and remorselessly tortures people to get a confession.
Guu from Haré+Guu is a definitive Creepy Child when she's in her deadpan mode (i.e. almost always), but only Haré seems to notice.
Any time you see a child in Hunter × Hunter, chances are he or she will be creepy. This includes the two main characters.
Gon, the protagonist, is a boy from a backwoods culture with an innocent heart. However, due to his Blue and Orange Morality outlook on the world, he can behave unsettlingly to those not familiar with him and his background. Not to mention that if anyone gets on his bad side (and he has a lot of patience with people he dislikes), he is downright frightening, such as him literally punching Neferpitou into a pulp.
Killua, the deuteragonist, is Gon's age. He freely murders people in bloody ways without batting an eye, as he comes from a family of assassins and thus does this regularly. That being said, he learns friendship and compassion from Gon as they travel together, so he becomes less creepy as the series goes on. He can still be quite scary though if he's angered, just like Gon.
And more recently, we're introduced to Killua's younger sister Alluka. When she asks you to do something, and you don't do it, she kills you and likely a lot of other people you know. When she asks you to do something, and you do do it, her eyes and mouth go completely black◊ but is otherwise harmless. Still freaky though.
When at her worst, Shokatsuryou Koumei (Zhuge Liang) from Ikki Tousen. She's a middle school Moeblob who never changes her expression and can devise the most amazing and shocking plans... She's mostly on the side of good, adores her leaderess Ryuubi and is nowhere as villainous or ruthless as her rival Shiba'i, but she's still creepy.
Marie Joseph from Innocent is an extraordinarily disturbed child, with a penchant for torture and execution that far exceeds the family heir's, Charles Henri.
Anju, Karin's younger sister, is an Emotionless Girl vampire who carries around a knife-wielding talking doll that is possessed by a serial killer. She later proudly shows Karin her collection of creepy dolls and asks if she wants to hear their history. In response Karin rather understandably lets out some Tears of Fear and promptly exits the room.
Ginger Bread is a vice-captain of the Millefiore Family's eighth squad, the enemies of the main protagonists in the future. He is also known as the Magician's Doll, primarily due to his usage of similar looking dolls that are clothed in a wizard's hat and cape, and have stars in their eyes (like he does), to fight. In addition, he uses spiders, summoned by what he calls sorcery. He claims to have had a hand in the death of Colonnello. As such, Lal Mirch engages him in battle, but after he loses, it is revealed that she was only fighting one of his dolls.
Mukuro Rokudo, when he was a child, as a result of his Dark and Troubled Past. He has a strong dislike for the Mafia due in part to the experimentation he suffered as a child at the hands of the Estraneo Mafia family. He killed his captors and he offered Chikusa and Ken, who were also victims of experimentation, a chance to join him in his quest to destroy the world. However, not only does he want to take revenge on the Mafia by destroying it, he also wants to cleanse the world of its filth by invoking universal suffering through a world war. Also, Mukuro was taken in when he was a child by a Mafia boss named Lancia. Except Mukuro betrayed and brainwashed Lancia with his Magical Eye which forced him to kill his entire family. And during his first meeting with Tsuna, he pretends to be a poor, innocent boy who was kidnapped by... Rokudo Mukuro.
Loveless is full of creepy children (and adults) with twisted pasts, but one that stands out is Kio's daughter, who looks ten, acts forty, is the head of the (very wealthy) family, and has no cat ears — meaning she's not a virgin, or possibly not a normal human seeing as Kio would have conceived her when he was ten.
Laetitia and Poupee from Madlax. By the end, though, Laetitia loses a part of her creepiness and is accepted by her "creator" Margaret, who adopts her as her little sister.
Masato from Rumiko Takahashi's Mermaid's Scar, the second compilation of the Mermaid Saga. At first glance he looks like an eight year-old boy, living happily with his mother... but then, why does the mother look so sad to be with him? Why did she try to abandon him? Why is she trying so hard to kill him? He's also pretty handy with Razor Floss and an axe as big as he is.
Also Sanetoshi's companions Souya and Shirase. These two Creepy Twins dress and look like clones, finish each other's sentences, seem to be completely devoted to Sanetoshi, don't say almost anything except singing praises to him with creepy and child-like smiles... uuuuuh.
Little!Himari skirts this line. Especially when she coldly tells little!Shouma about what happens to the Unchosen people of this world. Thank God she gets better.
Himari: "This world is divided into the Chosen and the Unchosen. To be Unchosen is to die.
Johan Liebert. Though most of the story takes place in 1995, in which he is a very creepy 20-year-old instead, he's been a sociopathic murderer and master of More Than Mind Control since he was 9. There is also Dieter, who has a genuinely creepy moment during the "Kinderheim 511" arc, despite being one of the most innocent characters in the story.
Also, Johan's twin sister Nina/Anna had trappings of this when we first see her.
Martin (from the episode A Child's View). While he may not be clever enough to be the next Johan, one could easily see him growing up to become like Roberto.
MW has Michio Yuki becoming this after inhaling MW.
Gaara starts out this way, thanks to certain well placed traumas in his past, as well as being the Soul Jar for the One Tails. Thank God he gets better.
Sasori is a milder version of this when he was younger, after his parent's untimely deaths. It just gets worse until he snaps. Unlike Gaara, he doesn't get better.
Itachi also qualified, being a gifted Child Soldier and The Stoic since he was seven years old. His father mentions that he's 'always been a bit strange.'
If the flashbacks are to be believed, and if his dark side is of any proof, Naruto himself was this when he was about 4-5 years old, at least until he decided he wanted to become Hokage and to stop feeling sorry for himself.
Rei I was definitely a creepy little girl, especially because her resemblance to the late, lamented Yui Ikari was much more pronounced at that stage. Practically everything about her made alarm bells go off in Dr. Naoko Akagi's head, and that's one of the reasons why she went crazy and killed Rei I, then herself..
There is an episode with Shido trapped in a well with a Breed and a child. Shido spends the entire episode trying to protect the "innocent little girl" but it turns out, the girl turned out to be a vampire, most likely an evil one.
There is another episode with a little boy who died, turned into a Breed and killed and mutilated pets and stray animals to feed off them. The child's mother, however, refuses to let Shido kill the Breed, convinced he is still her son... which leads to an even creepier resolution: since he can't do anything to convince the mother, Shido just leaves them alone.
When she was no older than 12, Silvana Greone aka Intoccabile from Noir scared the crap out of a pre-teen Mireille Bouquet when they played together, due to her unhealthy fixation with pointy objects. By the time she's in her 20's, she's a full-blown Dark Action Girl and still a thoroughly creepy Magnificent Bitch.
We briefly see Chloe as a child, and she was also pretty creepy. To not say of little Kirika, who killed Mireille's family before she turned six. And boy does that haunt her teen self when she recalls it.
According to Rinko, Yuuto from Omamori Himari was one of these when she first met him. He gets better though.
Tama ismanytimesworse. It doesn't help that she speaks with her mind, and that her side of any conversation is almost never seen.
The flashbacks to Donquixote Doflamingo's Dark and Troubled Past show him as one of these. At age ten he was so creepy that he scared the shit out of a mob torturing him and his family and ultimately commited patricide since he blamed his father for the utter shit the Donquixotes went through. If that wasn't enough, he then cut off his father's head and took it back to Mariejois, hoping that the Celestial Dragons would take him back after doing so. He was so evil that, as a child, criminals (his future executives) adopted him and made him their leader, just because of his "huge potential".
Past Law counts as well - he first meets Doflamingo when he breaks into his hideout, with grenades on his body and a huge slasher smile on his face and demanded a place in the crew in order to kill as many people as possible. And that was when he was ten years old. Luckily, Doflamingo's brother Corazon fixed him.
Mr. 3's teammate Miss Goldenweek is an emotionless girl who can manipulate people with colors. We never get any information on her backstory, motives, or even the reason why she's in a criminal syndicate despite looking like she's eight - but the fact that she can calmly have a tea party while others slowly die nearby is definitely unnerving. However, she does care about her Baroque Works coworkers, as a cover story shows her and some fellow members trying to rescue the ones who got arrested in Alabasta.
One of Doflamingo's executives, Sugar, looks like a little girl in a bear hoodie. However, she can turn people into toys, thus forcing them into slavery and erasing them from the memory of everyone who knew them. Not only does she not feel any remorse, she gleefully remarks that this ability can bring her an army of slaves, and she knows how to manipulate others using her appearance. Her real age is unknown; she actually only looks like a child, because her ability stops her from aging.
Rob Lucci murdered 500 soldiers who got taken hostage, and then the pirates that captured them, single-handedly when he was thirteen—scene of him standing on a mountain of corpses included. That gets even scarier when you realize that was his job and, since he was the one assigned to it by the government, he must have already proven himself enough to get the job...meaning that he was already an experienced assassin at the time.
Leo, post-FaceHeel Turn (sort of), at least when he was "trying" to fight Oz.
Lacie as a child. She intentionally cuts and licks Jack's ear while trimming his hair, and later, after slaughtering two slavers with her chain, the only comment she makes as she stands amidst the carnage is, "Look, Jack! Blood red rain!". As an adult, however, she seems to have mostly grown out of it.
Phillipe West after his father's deathis seen in one panel talking on a disconnected phone to his father.
Sabrina, the psychic Gym Leader from Pokémon with a nasty habit of turning people into dolls. Actually, the real Sabrina (as seen in the games) is older, but her childhood form still wanders around as a doll bouncing a ball. (Sabrina's father is a little creepy himself, but then, she inherited psychic powers from him too, apparently.)
In the movie Pokémon 3: Spell of the Unown, Mii/Molly is an innocent child whose mother left due to her husband being obsessed with studying the Unown, and Molly's father is taken by the Unown themselves. While playing with the Unown tiles, she summons the Unown, who create an illusion Entei. She proceeds to use her innocent desire for friends and parent figures to have Entei kidnap Ash's mom and brainwash her into think that she's Molly's mother, get a LOT of the countryside covered in weird crystals, and, when Ash tries to get his mother back, she spawns waaaay too many of said crystals. In spikes. To make matters worse, she ends up being able to control the Unown, and ends up as a little girl in a grown woman's body trying to be grown up. She even has three crystal Pokémon, which are all based off three cute mons, but are all insanely powerful.
On one hand, there is the no older than 13 Touga Kiryuu. While Nanami's memories have him as a real Nice Guy (and according to Word of God he was more or less nice in his super early years), the flashback to him and the also young Saionji stepping into the funeral of Utena's parents has him casually touching little Utena's hair as she's curled up in a semi-open coffin, showing next to no empathy for her plight (unlike kid!Saionji), and having already signs of being a Manipulative Bastard in training.
And on the third and final hand, there is Nanami herself. When she was a child, she gave Touga a kitty as a pet, and when the kitty took his attention away from her she killed the kitten via stashing it in a cardobard box and placing it in the near stream. (It's implied that she didn't actually understand what this truly meant, even as the kitten "left", but it's still creepy.)
The Amazons Quartet. It is not known how old they really are, probably not older than 13-14, but they tend to act very childish and apparently find battles against the Sailor Team "fun". Yet, their tactics aren't anywhere near innocent.
Anime!PallaPalla is an extreme example. More often than not, she's a Third-Person PersonBratty Half-Pint who often cries, throws tantrums, and demands to be entertained by her "sisters"/teammates. Then comes the episode with the dental clinic, where she declares that the best way to cure cavities is to rip the head off (which she demonstrates on a doll, creeping the fuck outta the other girls). Then the anime episode where she controls the senshi using an enchanted doll to make them repeat the doll's movements, and then she nearly tears said doll apart by its hands. And we also have the episode where she attacks dozens of people (the other girls tend to attack only one person), rips out their Dream Mirrors, and then cheerfully claps and sings Where, oh where is the Golden Mirror ~ before Moon and Chibimoon's arrival.
Souma's 14-year-old younger sister Sakurako was already plenty creepy as a teenager, but when she's seen as a little kid in a flashback... oh, boy. She appears pointing at Souma with a blank expression on her face right after Katsuragi force Souma to kill Sakurako's mom for abusing him, and later she appears carrying around an oversized kimono, peeing herself and giggling madly during a cherry blossoms watching session.
Souma as well. In the first scene, he was barely 11-12 years old, and with his cynical smile he creeped the shit out of a detective investigating Lady Saiki's death. And then the audience learns why he is that way...
Serial Experiments Lain: Lain Iwakura and others as well. Like the one chasing people through the virtual reality game or that girl who commits suicide in the opening. This anime takes the cake for creepy children.
In Slayers, Hellmaster Phibrizzo, Man Behind the Man to Chaos Dragon Gaav (who is, oddly, another Big Bad but not The Dragon to Phibrizzo) and one of five ancient demon lords, takes the form of a ten-year-old child in every appearance.
In Evolution-R, Rezo the Red Priest in Pokota's human body.
Soul Eater's Crona flip-flops between being the Creepy Child and The Woobie. Also, little Rachel after Medusa takes over her body. Stein was almost certainly this as a child/young teenager, as a flashback shows him casually beating up a man while ignoring Spirit's complaints.
The Emperor becomes one of these for a short while in Soul Hunter. Blame Dakki's influence and experiments, but he's pretty damn creepy as a kid. Doesn't help he's also been turned into a superhuman weapon.
Ichise. As a child he was perpetually emotionless, even when he and his mother come across the body of his mutilated father hanging from the ceiling. While his mother cries at the sight he just looks at her with a confused expression on his face.
Kano. Although the series doesn't show his childhood firsthand, it's made amply clear that he was an extreme Creepy Child who never really grew up, always wanting a bigger playground and more real people to play on.
Koon Agero Agnis from Tower of God used to creep the servants of the Koon family out a lot as a child, probably because he could easily see through everybody.
Zazie the Beast, in the anime version. Unless he's actually some kind of midget.
Also from the anime, young Knives. He started out just Vash's more serious and mature twin brother, but even before he starts his Evil Plan to wipe out the crew and crash the fleet he has started to give off some seriously creepy vibes.
Manga young Knives, in a very different situation, manages this briefly right before going Axe-Crazy, with his Dissonant Serenity after The Reveal. He's faking Easy Amnesia. Until then, manga Knives is the sweetest little thing, who really believes that everyone can understand each other and everything can be worked out because everyone's hearts are the same.
Manga Legato, in his back story. I mean, yeah, when he's having his weird Meet Cute amid the rubble with Knives, then it's just the saddest Hope Spot ever, but before they found out his plot and started raping him to death he was this kid putting off his bid for freedom to be sure of being able to kill absolutely everybody he had ever met with his mind. Have we mentioned this is a Crapsack World?
Miyu. She's got the excuse of being mentally an older woman who was allowed to keep a teenage form to capture the Shinma.
The little girl form the first OAV, Aiko, also qualifies. She appears as an Ill Girl in a Convenient Coma, but later a flashback shows that she sold her soul to a Shinma after her parents died in a car accident where she was gravely injured and, out of trauma, she started calling herself a vampire since they donated their blood to her to save her life, right before perishing. She's both creepy and terribly pitiful.
Mokuba's original form in the manga was pretty bad, too. Especially given he forces Yugi to play a rigged game with him, and until the Other Yugi comes out it's a fifteen-year-old who seems like a little kid facing a ten-year-old with the mannerisms of a mafia boss. And that homicidal grin when he was cheating at roulette to poison Jounouchi and Yugi to death...
In the anime, there's Noa Kaiba. He died at age 10 in an accident, which is why Gozaburo adopted Seto and Mokuba. But his conscience was trapped in a virtual game, which he then uses to get revenge on Kaiba for replacing him and on Yugi and his friends as well, using all the traps and manipulations at his hand to get it done.
In Yu-Gi-Oh! The Dark Side of Dimensions's prequel manga, Sera has knowledge beyond her years, and claims to hear voices from the netherworld. She's also a skilled manipulator, challenges Kaiba to the technological equivalent of a Shadow Game, and is working with Aigami. In the movie itself, though, she's a hero and tries to stop Aigami's path to darkness.
In one episode of Yu-Gi-Oh! 5D's, Rua went up against a ghostly child named Michael. Michael - but not Claire - was ignorant of the fact that they were dead, and Michael was obsessed with protecting his sister. Fortunately, losing a duel to Rua set things straight.
Hiei from Yu Yu Hakusho was a really creepy infant. Small pupils, sharp teeth, and a Slasher Smile. Well, he's a demon after all. And when he grew into a kid, he was a Blood Knight who killed other demons for fun.
Shiho: "The pain and hate of the fish which were brutally murdered... it's delicious."
In a flashback arc, we find out that before she and Aoi first met Kaoru (at the age of 5) Shiho was even more sadistic. One of her favorite ways of torturing Aoi was making crime scenes from Aoi's dolls. (In which she was creepily creative!)
Hyobu has his moment, when he suggests shooting an enemy pilot in a forehead as the quickest method to deal with his plane. He is 8 at that time.