7 Seeds has Natsu from Team Summer B. She's incredibly shy and she doesn't like confronting people, so she prefers to be nice and agree with them, regardless of what she might actually think or even like the person.
Matsuri from the same team, to a lesser extent, is Depressed. She wanted to have fun and hence came off as a very shallow person, with no real friends, so she's been lonely.
Farnese from Berserk. She used to be a spoiled, pampered Lonely Rich Kid that had what she needed to be happy. In appearance. It appears she was actually a Pyro ManiacEnfante Terrible who was suffering from her parents' Hands-Off Parenting and was called the "demon child" of the Vandimion house, making her a combination of Depressed and Mentally Unstable.
Elizabeth of Black Butler admits in one chapter that her cheerful and Genki Girl personality is partially a mask to cheer Ciel up, who has some serious issues himself. She also has insecurities about being a cute, perfect wife to suit Ciel especially since she has a hidden badass side. She once comes across him while he's sleeping and cries "I'm not good enough..."
Mato Kuroi of Black★Rock Shooter is a major one. She hides her pain well and focuses all of her attention on fixing other people's problems rather than acknowledging her own, to the point that she starts denying she even has any issues.
Orihime Inoue from Bleach is a very gentle, sweet girl as well as the local Genki GirlBarrier Warrior. Also has huge self-esteem problems, views herself as inferior to the Shinigami, feels troubled because she adores said Shinigami (and specially Rukia) and is in love with Ichigo. Cue to her almost having quite the breakdown in the Arrancar arc... and a full-blown one in the Hueco Mundo arc.
Captain Unohana. Not only is she the First Kenpachi, she is also just as Ax-Crazy as the current one. She named herself Yachiru due to having mastered all known sword styles, and was, and still is, considered to be the most diabolical criminal to have ever existed in all of Soul Society. The braid she usually has her hair in is to hide the scar of the only wound she has ever gained, inflicted by Zaraki, while the scar on Zaraki's eye was inflicted by her.
That being said, her motherly persona we've seen for the most part is implied to be as equally genuine as the Ax-CrazyBlood Knight side of her personality she's been hiding for centuries.
Also, Ichigo's mother Masaki was revealed to be this before meeting Isshin. Despite her bubbly and energetic appearance, she was very not happy with her current situation.
Anko from Bokurano spends some time Depressed, after she's chosen as next pilot as punishment for hitting The Mole Youko, her dad is caught cheating on her mom, and Mrs. Tokosumi goes Lady Drunk mode out of despair. With the help of her friend Kanji, she gets better, just in time to fight and die in peace.
There's also Takako Shimizu from Chobits, Shinbo and Hideki's Cram School teacher. She always smiles and helps them out, but hides how her husband ignores her in favor of his persocom...
Mary Magdalene in Chrono Crusade almost constantly smiles, even though she's constantly having visions of the future and the past, some of them horrifying and violent. In fact, she has so many visions she has no memory of her childhood or even her name—except for a reoccurring vision of Chrono killing her with tears streaming down his face. She's even smiling when she dies.
Sanae Furukawa from CLANNAD maintains a cheerful exterior even after her daughter Nagisa dies giving birth to her granddaughter Ushio. When she finally cries five years later, it's truly heartbreaking.
Corpse Party has Seiko, who smiles and strives to keep everyone cheerful despite being in a lot of pain following her mother's disappearance. The mask winds up breaking upon being accused of not taking anything seriously.
Most of the members of the "Scratch" cult in Cowboy Bebop. Faye's smile while talking about getting rid of her debts by getting rid of her body is especially unsettling...
Death Note: Misa. She acts happy and cheerful but Beneath the Mask she is psychologically traumatized, uncaring, self-destructive, manipulative and genuinely believes that criminals should die.
Digimon Tamers had a Stepford Smiler in training in Juri Katou. Early on, her main character traits are being cheerful and carrying around an alternatively cute and creepy puppet; later in the series, we find out that she never quite got over her biological mother's death, and everything went to crap as soon as the series' running "joke" happened: for non-fans, Leomonalwaysdies, and this time, he just so happened to be Juri's Bond Creature.
Digimon Adventure tri. takes it Up to Eleven with every single Digimon partner of the Digidestined in the third movie Confession. After realizing that they're all slowly succumbing to an infection that will drive them mad and start hurting others, they are informed that the only way to fix is to wait for some higher being of the Digital World to trigger the 'Reboot' process to remove the infection, that has a side effect of removing all their memories. Not wanting to inform their partners of this terrible truth, they try their best to act cheerful in front of their partners while doing happy and nostalgic things with them with what little time they have left. Only Patamon, whom Takeru already knew is infected, and Agumon, who goes overboard with the act that he accidentally reveals to Taichi that something is wrong, are only able to share the grief with their partners.
Mio Hio from D.N.Angel. Her initial personality of being a smiling Genki Girl is not her true personality. She's actually really depressed because she's actually a voodoo doll taken human form and she doesn't have much time left as a human being.
Tohru Honda from Fruits Basket is a genuinely kind and caring person, but has serious insecurities that she hides behind her smile and cheerful disposition to avoid troubling her friends.
Mitsuki Koyama, the main character of Full Moon o Sagashite. She acts like a Cheerful Child whose biggest wish is to see the boy she loves again, but it's eventually revealed that the boy she loved had been dead for years and Mitsuki was only pretending he was alive to everyone else, so that she wouldn't have to face that reality.
Mercedes Morcerf of Gankutsuou tries to maintain a perfect appearance and smile and loves to pretend that everything is okay even when everything really isn't.
Also Heloïse Villefort. She seems perfectly normal in her first few appearances, with a warm motherly appearance and pleasant smile. She is the stepmother of Valentine and is the second wife of the crown prosecutor Villefort. She is the biological mother of her young son Edward, from her first marriage. Valentine is to inherit all of the fortune, leaving her stepmother jealous because Edward doesn't get a single penny. So when the Count seduces her and innocently introduces her to toxicology and gives her a deadly ring which releases a deadly poison Heloïse becomes murderous and tries to poison Valentine and her husband, as well as inadvertently almost poisoning Albert and poisoned one of the servants of the Villefort household. And she does all this while maintaining her angelic motherly facade up till near the end of the series when her husband finds out what she's been doing and points out that she's nothing more than a murderous insane woman. She tries to deny his accusations before she breaks down and collapses to the ground, finally giving in to her insanity. Her husband then puts her in an insane asylum for the rest of her life.
Miho often acts like this when she's feeling troubled. In Little Army, after her mother arrives, and her sister suddenly turns cold toward her and walks off, she smiles to her friends and insists that she's ok, but none of them believe her. In the main series manga, after hearing that she will be disowned if she loses against Pravda, she acts as though nothing happened the next day, and Yukari, who overheard what Miho had heard, makes note of this.
Anzu turns out to be one in light of the fact that she knows that the school will be shut down unless they win the tournament. Her facade only slips when discussing that.
Asakura Ryoko from Haruhi Suzumiya fits this trope more or less. As she acts like a Purity Sue, she keeps a cute, caring and friendly appearance, even when she is about to stab you with her beloved combat knife, or when dying. Since she is an Artificial Human, unable to empathize with humans, it is very likely that it is just a fixed masquerade, thus Unstable.
It becomes especially disturbing when you consider that it's likely the only time only time she's really smiling is when she's trying to kill Kyon. She's also, arguably, a deconstruction of both the Stepford Smiler and the Purity Sue. How is it possible to never stop smiling for a moment? To have no understanding of what the expression means.
In the Disappearance movie (spoiler warning, it shows a scene from the film), that smile seems even more sinister than it did before.
Like Misao, Tsubomi Hanasaki of HeartCatch Pretty Cure! is Depressed. Her parents were once major workaholics and, despite the loneliness, she had watched them go off with a smile. This went on for years and years until she finally broke down upon seeing a mother bird tending to its young. It was just pure luck that her parents had returned for notes they forgot in time to see their daughter break down and eventually changes job from botanical researchers into normal flower shop owners to make up for the lost time. The rest of the series shows her growing up from this incident and becoming a much stronger person.
Nanami in episode 14 as well, who hides the grief from losing her mother under a perpetual smile, believing "A smile is the most important". She eventually loses it when her sister snaps and believes her to be uncaring, leading for her heart to be released and stolen by Sasorina.
Seras Victoria from Hellsing. She's unnaturally cheerful and goofy for someone who's had all of her police partners killed, being nearly raped and killed by a vampire and then is actually turned into one. That's just the first volume/episode. Also her parents were killed in front of her and her mother was raped. In that order. The manga was a bit more subtle about her being a Stepford Smiler but the OVA series comes right out and says it.
Rika. She has been murdered and revived countless times throughout the series, and retains the memories of every death she has ever suffered. This has resulted in a mix of types - Rika is revealed to have lost pretty much all hope of saving herself from this brutal cycle, becoming very depressed and hollow inside as a result (she even gets drunk on occasion - this is a girl of about 10 years old - to drown out everything). Despite her struggles, Rika keeps a brave face, and constantly acts like the sweet, cute Cheerful Child that everyone expects her to be, to avoid freaking her friends out if anything (she does break this pretence at times to scare her killer(s), or to convince others to listen to her).
The series' main villain, Takano Miyo, also counts; she constantly acts like a friendly, helping hand to the arc's protagonist, sweeter than sugar, only to reveal herself as a cold Magnificent Bastard with one hell of a God Complex in the final arcs, who guns down anyone willing to mock her goals and beliefs.
Underneath Rena's cheerful facade, and obsession with all things that she believes are cute, lies a girl who is utterly broken over her parent's divorce, tried to kill herself in the past, and lives every day with a smile just to make others believe she is happy.
Mari's mother Eriko is Unstable. At first she seemed like a nice, normal mother however it's eventually revealed she is neglectful and has control issues. She went as far as to change her daughters name from "Fumiko" to "Mari" as a child to disassociate her from the recently deceased Doting Grandparent she loathed.
Chikane and Himeko both from Kannazuki no Miko. Himeko had a really terrible childhood and tries to hide her insecurities. Chikane is constantly trying to hide her feelings of love for Himeko...and eventually remembers that she killed her in a past life for a ritual to rebirth the world. Because of this, she was unable to help in the ritual to summon Orochi's nemesis so she wore the mask of the villain at this point in the series when she realized the reason why she couldn't help, because a deep part of her still hated the god. That was Himeko would become stronger and kill her to complete the ritual.
Hino Kahoko of Kin'iro no Corda following her Break the Cutie process tends to smile and act like everything is alright around her friends but if she's alone she tends to lose her smile and get depressed.
Kyouran Kazoku Nikki has at least 2. Kyouka, the self proclaimed God in Human Form, is actually a banished princess from a race of sentient creatures from a place called Shangri-la. In episode 26's Crowning Moment of Heartwarming of the entire series, she boldly proclaims that she is the child of Enka, and must be killed. She breaks down, claiming that she doesn't even know who she is since her people are effectively a race of sentient beings that possess you and give you animal ears, lending evidence to her being Empty.
Yuka is one as well, since, despite the fact that she smiles all the time, she was a "silent doll" for a large crime family, which means that everyone in the family abused her, both physically and psychologically, and it was her job to take it all with no reaction. One would think that this would make her Empty, but in actuality she is Depressed and keeps smiling in order to not burdenher family.
Hayate of Lyrical Nanoha shows signs of being depressed when in her Ill Girl phase, as Shamal notes that she hides her pain from those closest to them, and while she often comes off as cheerful, she confesses in the second Sound Stage for the season and the start of Episode 9 that she realizes that she might die soon, and for a long time, was not scared at all because of how lonely she was. Note that she was a nine-year-old paraplegic orphan at the time.
Shamal implies that Nanoha was this during while she was injured and when her recovering her ability to walk or fly was in doubt.
Mai Tokiha from Mai-HiME keeps a smile on her face almost constantly because she doesn't want to burden any of her friends with her personal problems, which include a dead mother and a sick younger brother.
The anime version of Shizuru: She manages to keep up a cool and semi-cheerful facade for the vast majority of the series, all while trying to content herself with being Natsuki's friend despite believing that she can't tell her how she feels. We don't see it break until Natsuki rejects her.
Izumi Maki constantly amuses herself by telling bad jokes and laughing at them alone in Martian Successor Nadesico. While she appears initially as a Cloud Cuckoo Lander, she is depressed and the puns are designed as a coping mechanism to distract her from the pain of losing two separate boyfriends.
Natsume's mother from Master of Martial Hearts spends all of her screen time Depressed, because she used to be your average plucky Action Girl, until the took part to a Street Fighter expy for a MacGuffin supposedly able to grant wishes to the winner. However, she wasn't warned beforehand of the Fate Worse Than Death awaiting the losers, so she was mindraped into helplessness, got her voice box removed and sold into sexual slavery. Upon escaping she found out that her younger sister, another Action Girl in tag team with her, was raped and beaten to death by the very man organizing the tournament... who also had married the winner (the indirect responsible for her tragedies) and sired a daughter as strong as her mom. She stays a smiling Cute Mute until the reveal, then is shown scratching a picture of her former self and her dead sister with nightmarish expression. Until worse happens...
Yuri Tokikago turns out to be one as well, likely Empty due to her strong self-hate and hidden issues related to the (currently undisclosed) "secret" she has in her body. Also has Unstable traits, considering the end of episode 14...
The girls from St Arsenal Academy in the manga Mission! School. At first they seem like your average happy schoolgirls. However, it turns out that they have been brainwashed and when they are provoked are revealed to be highly trained killers. The scary thing? They never stop smiling, even as they are taking on their enemies.
Kaworu Nagisa seems to fit this trope as well. He's always smiling, but in reality, he just wants the world's end. It got better for a while, but shortly after, it all goes to hell.
Also, Ryouji Kaji, who was smiling calmly in the very moment he was shot to death
Shinji: For most of the series he walks around quietly with a little smile on his face, even when it becomes clear he's dying inside.
In New Game!, Rin states that Kou used to be pretty serious when she first came into the company, and her current funny personality is a way to cope up with all the bullying she had experienced at work.
Koala from One Piece is Depressed: During her time as a slave, she had to put on a smile at all times, because slaves who stopped smiling or cleaning would be killed. Needless to say, that smile hid. The poor little girl was so scarred by her time as a slave that she kept the Stepford Smiler façade even after she was freed. Fisher Tiger finally broke that façade by altering her slave mark to make it look like a sun, and telling her that it is OK to cry.
Dawn from Pokémon, surprisingly enough, fits this trope. This is actually foreshadowed from a very early point - pretty much her first episode - and hits its nadir with her losing streak, where she fails to qualify in the appeals round two times in a row. She tries hard to put on a happy face whilst not having the confidence to enter another Contest in a while, which she fails to upkeep. She finally overcomes the act when she wins the Wallace Cup (beating May, a more seasoned opponent). It was part of her Character Development - given how hard she tries to live up to her mother's legacy - and she comes full circle when she's happy with 2nd place in the Sinnoh Ribbon Cup.
Magical Project S's tough Misao Amano is the Shrinking Violet. While she appears to be cheerful, in reality she's hiding her lack of self-esteem— and her loneliness and jealousy which manifest as PixyMisa.
Way before her, Sasami in the Tenchi Muyo! proper. Imagine going 700+ years thinking that you were not the real Sasami, that the real one died when she was four, that you're nothing more than a vessel for a powerful goddess and that, if anyone else found out, there was the humongous chance that everyone would turn away from her.
To say nothing of Anthy. She is initially presented as a demure, somewhat shy and even submissive girl, but by the end of the series it has been revealed that not only has she been living with an unimaginable amount of both physical and emotional pain, but she could even be considered a puppet of the series' Big Bad (her own brother).
Utena herself shows signs of being one at times. She was rather depressed in the past to the point of wanting to die after her parents died because she saw life as pointless if all humans eventually die.
In Sailor Moon Stars, Usagi/Moon slowly becomes one of these to hide her worry because Mamoru hasn't come in contact with her ever since he left to study in the USA. She thought he was working too hard in university, but unbeknownst to her, poor Mamoru was dead through Bus Crash.
In the live-action Pretty Guardian Sailor Moon, a somewhat similar theme's explored when Usagi learns from Mamoru that her own negative emotions are empowering Metaria and helping bring about The End of the World as We Know It, and so remakes herself into a Stepford Smiler. After Minako has died, Usagi tries to greet him with a teary-eyed, obviously fake smile, and when he tells her it's okay to cry now, she practically falls into his arms in tears.
Minako could have become one at the end of the Codename: Sailor V manga. In the final chapter she recovered the memories of the fall of Silver Millennium, discovered she's about to embark in a battle that may well end with her own death and bring The End of the World as We Know It, her one true love was revealed to be the Big Bad of the series and a subordinate of Kunzite before dying at her own hands, received a prediction she'd never find love due herself choosing duty over it, and renounced her dream of becoming an idol... And yet, as she returns to Tokyo she smiles cheerfully.
This is made even more apparent in Sailor Moon Crystal as she cheerfully greets the Senshi after the battle with Zoisite and sweetly accepts their offer to help in fighting the Dark Kingdom. When they leave, she turns to Artemis, the smile abruptly fades, and she reassures Artemis that she would be taking on the Dark Kingdom alone.
Teru Miyanaga of Saki puts on a polite face for the press and public in general. Sumire chides her for this, saying that her "salesman smile is absolutely cruel," and while Teru insists that she's not completely lying, Sumire points out that sort of behavior is too different from Teru's actual personality.
It's plainly obvious that the overly optimistic and cheerful Kafuka Fuura from Sayonara, Zetsubou-Sensei is a Stepford Smiler. Even though she hides her traumatic past, some of her classmates are acutely aware of her mask and are mortally terrified of her. It doesn't help that she's secretly stalking Itoshiki-sensei by disguising herself as a college student who lives next door and has been steadily poisoning him with gifts of food...
Most of the cast of School-Live! are this. For a good portion of the series they are stuck in their school surrounded by zombies. They do have their genuinely happy moments but it's not exactly easy being cheerful when you're struggling to survive with the odds against you. The only reason Yuki is constantly happy is because she hallucinates there's no zombie apocalypse. According to Yuuri she was depressed and anxious until she blocked everything out.
Seraph of the End: Shinoa shows shades of this, due to her friendless background and history of abuse from her family.
Kyouko Mogami from Skip Beat!, due to having had an abusive, unpleasable mother, and later living and working in a ryokan (Japanese inn) where she picked the attitude of "clients shouldn't see you upset ever." Any time her real feelings and temper come to surface, she managed to put them back quickly, to not disturb the people around. Kyouko only breaks with this when she is properly informed that all her efforts will be not rewarded at all, and then realizes that living like she was constantly tending clients neither does her any good.
Complicating Kyoko's case is that after she discovers her "prince" Shou's heartless betrayal of her and snaps (in the first chapter), she continues from there. And the skills and habits she learned in her previous life and her tendency to live for others coexist really strangely with her new life goals. She wouldn't have made it so far if she wasn't the person her childhood made her, but she had to break with her childhood to go anywhere.
Really complicating her story? She has had a truly, deeply miserable, loveless life, but she always kept her spirits up. On the other hand, her Orphan's Plot Trinket was received ten years ago from a talented ten-year-old Hollywood Brat whose main sources of suffering were "I can't live up to my father's legend" and "My parents (who adore me) are busy a lot." A psychic exposed to the rock reports that the pain it absorbed from that kid was about a hundred times worse than what Kyoko's pitiful life has put into it. So apparently she really did keep her spirits up, and wasn't Stepford Smiling that much?
She was, but it turns out that kid's past includes guilt over maybe having killed someone so she wasnt overreacting.
The over-enthusiasticprincess Amelia from Slayers, if one looks deeper, is Depressed; as a daughter of a prestigious and powerful royal family, she is subjected to horrific violence and carnage as her family battles over the line of succession to the throne. Her Missing Mom was murdered, in-series she sees one of her uncles and her cousin get killed attempting a coup, her sister has long since run away, and her father (the current heir apparent) is a go-to target for assassins. It's summed up well enough in the Light Novel she's introduced in:
"But you know," [Amelia] added, still sporting that strange smile, "I really don't like that you can't trust people in your own family around here." It could've been my (Lina's) imagination, but just then I sensed a great sadness in Amelia behind that bubbly facade. There was definitely more to that girl than met the eye.
Everyone in SoilNew Town but especially the missing Suzushiro family who remain cheerful in the face of vicious harassment and continuously deny that there's anything unusual about them to their daughter. For example they insist she has a birthmark on her back despite it being on her chest and they ignore the fact that her memories of her old town are off (EG "I lived on the third floor" - building has two floors - "school was behind the supermarket" - the only thing behind the supermarket is the edge of a cliff). It's heavily implied they're from another dimension.
As for everyone else: the housewives of Soil harass the Suzushiros after they get them into a pyramid scheme but the real instigator is the town president, an obsessive neat-freak pedophile dentist who raped all their sons (literally every boy aged 9-10 for at least the past 9 years), used the secret video cameras he put all over town to blackmail the housewives into harassing the Suzushiros, used a drill to torture one of his victims' mother into silence, and killed a bunch of cats to intimidate a homeless woman because her appearance violated his sense of "purity".
Yoko Nakajima, the main character of The Twelve Kingdoms, starts out the series as a junior version of this in regards to her home and social life; she reveals later the fear that there was truly nothing inside.
Dita from Vandread was surrounded by people who were depressed often as a little girl. This, in turn, depressed her. She tried to cheer people up by acting as a ditzycutie, it worked, so the habit stuck.
Misaki from Welcome to the N.H.K.. She appears to be an innocent, religious girl that wants to save Satou from being a hikikomori, but the truth is that she's searching for someone more worthless than she believes she is herself and help them to give herself a sense of purpose. Megumi also has elements of this, telling Sato she's successful and happy when she's actually gotten caught up in a pyramid scheme while desperately searching for a cure for her brother, also a hikikomori.
Masane from Witchblade has this to a degree. After losing all her memories in the great quake, and even her old name, all she had to builUtrd a new life was her daughter Rihoko. She is constantly saying that Rihoko is the only thing she has and aside from being her mom, there really isn't that much to Masane. Well until she activated the Witchblade and became an Action Mom. As the series progresses, she gets better.
Himawari Kunogi from Xxx HO Li C is a lovely schoolgirl best described as concentrated cute. She later reveals that she suffers a curse that makes her a magnet for tragedies, and everyone around her dies horrible deaths or at least suffers terrible accidents, ever since she was a little child. And she tells this story with a smile on her face.
Tetsudo "Poppo" Hisakawa from Ano Hana. He's a happy, cheerful guy who loves to travel the world, but that is to cover up the guilt over his inability to save Menma from drowning over 10 years prior to the main story.
Mickey Simon from the Area 88 manga and OVA. He veils his war trauma, cynicism, and regret with a friendly exterior.
Prussia in the Christmas strips is "So happy being alone." Really.
Lithuania's a minor case. He can show fear and frustration from time to time, and he's genuinely a really nice guy. But he'll often be smiling and laughing while Russia is stalking and abusing him, or when the girl he loves is breaking his fingers, or when his boss is overworking him, or when Poland is giving him severe stomachaches...he also has No Sense of Humor.
For that matter, Poland is hinted to be this as well. He acts bubbly, cheerful and silly around his friends, but he actually has extreme anxiety around strangers. He doesn't seem to be a Chew Toy like Ukraine and the Baltics, but it's clear he is no luckier than them.
APH Dark Fic often interpret characters as fitting this trope, especially the most loudly cheerful and dense ones like America, Italy, and South Korea, considering that just about every single nation has had their fair share of brutal historical events in real-life.
Since APH is based on world history, (with one of its initial lead characters being freaking Nazi Germany), all its characters are all prone to act like sociopaths from time to time, because the community of nations has never, ever been civilized or characterized by good faith.
Considering the Lighter and Softer tone of the manga, however, this is greatly overblown by fanon, which in its tries to accommodate canon to more Real Life-like history ends up more often than not making the characters fall victim to lots of OOC behavior rather than finding a medium.
In America's case, he does play it straight when around Russia, responding to Russia's behavior by acting similarly; America keeps his goofy smile even when showing aggression towards him (unfortunately, fanworks tend to overlook this, often having these two scowl, smirk, or genuinely smile at each other instead). It's possible he may act like this to all his enemies.
Griffith of Berserk is an excellent commander, a masterful swordsman, graceful, elegant, and charismatic. He's also got serious problems, mostly involving his greatest warrior Guts. When Guts leaves the Hawks, it starts a downward spiral for Griffith that would culminate in the betrayal of all the Hawks in order to become Femto during the Eclipse as well as his monstrous rape of Casca.
Not to mention that this trope is played eerily straight after the eclipse and Griffith reincarnates himself into the real world with a brand new human body. Lacking empathy and just emotions in general, the most that Griffith ever gives to display any form of emotion is a soft smile which only serves to give the effect that he's constantly giving you the Kubrick Stare. It's amplified with his unsettlingeyes.
Yuki Giou of Betrayal Knows My Name clearly has some issues of self-worth stemming from being abandoned by his parents when he was real young. However, if someone should ask him if he's doing alright he usually gives them a cheerful smile and says "nothing's wrong" but tends to get depressed if he's by himself.
Add to that that apparently he served Aizen for a hundred years just to be in position to betray him at the last minute in order to avenge Aizen's wronging Matsumoto (he stole part of her soul) when they were children. She doesn't remember and moves on. He doesn't say anything and keeps his revenge. And he waited waaaay too long. So add a couple pounds of psychosis and Starscream to that description.
Hirako Shinji, too - he's hardly ever seen not smiling, and when he isn't it's because he's either really angry or something unsettling just happened. That man has been through a load of crap, and is probably in need of some counseling. Not as much as some other characters, but still...
Kisuke Urahara is probably one too, in a similar vein to Shinji. Sharing many of the same experiences, he carries himself with an enigmatic smile most of the time in public and can even act ridiculously goofy at times. Inside, he's really quite broken and when the smile fades he has many an introspective moment. Having to play The ChessmasterManipulative Bastard with everyone for 100 years - which means acting quite ruthless towards people he'd otherwise care about For The Greater Good - and distancing himself from all save his closest friends really doesn't help. It must especially sting because Kisuke was a Nice Guy before Aizen ruined his life.
In Blue Exorcist, we have Yukio. Smiling, nice, accomplished and smart...recently has been slipping and showing his insecurity, jealously, and hints of a far from stable mind set. On top of that, his own brother Rin, who is cheerful, book dumb, and loyal to a fault, has been showing signs that notalliswellinhisworld. The latter is worse than the former; while Yukio is pretty good at hiding his issues, Rin outstrips him by far, being capable of smiling and pretending things will be alright even when they definitely won't be and he's internally wondering why he is still alive. See the Impure King arc for that prolonged incident of fake smiling.
To wit; Ryuji knows Rin is faking things but thinks he's just being optimistic. In other words, even if you clue into him faking his smiles you will probably just think he's doing his best to remain positive. This isn't a lie, but is far from the whole truth.
And, more recently it turns out Renzo is one as well. What was originally funny is now very tragic, as it turns out he was probably abused by his family, has been put into too much pressure into being what everyone else wants, and is now stuck working for a bunch of psychos and Well Intentioned Extremists in the Illuminati. He's even worse than Rin and Yukio combined, keeping a cheery tone and facial expression. Unless one pays attention to his eyes one might very well buy that he doesn't care.
Ryousuke from Bokura no Hentai dresses up as his deceased sister to help his mentally ill mother. He hides it from all his friends besides Marika and Tamura and acts like everything is fine. Even prior to his sister dying he had issues due to being The Unfavorite.
In Brynhildr in the Darkness, after Kana has a vision of Neko lying dead with Kotori standing over her smiling, they instinctively believe Kotori is an Ax-Crazy murderess come to kill them all. She's actually a combination of this trope and Extreme Doormat, and is merely fulfilling her friend's dying wish that she keep smiling no matter how sad she feels; the real danger is Kikuko.
Okami in Code:Breaker initially seems to be successfully pulling Obfuscating Stupidity on his class with his Nice Guy persona. However, one chapter has several classmates wanting to cheer him up, commenting that he seems to be very sad but putting on a happy front for their benefit.
The lead... no, all of the Code: Breakers, seeing as they're super-powered Ax Crazies put to good use by the government (or something).
Oh so many people in Code Geass. Lelouch Lamperouge and Suzaku Kururugi, to start. Lelouch is Empty (seems kind and cheery, but is hollow and futile inside), at least initially, and Suzaku becomes Depressed (seems kind and cheery, but feels sad and depressed) over the course of the series.
And Rolo Lamperouge. He seems like a perfectly normal and very nice kid ... until people start dropping dead around him. Not to mention he can be considered Unstable due to his Cute and Psycho and Yandere tendencies.
You Takami from Deadman Wonderland seems to be one of the friendlier people in the prison/themepark, but he's not above stealing the antidote for Ganta's poison, taking someone's entire ear for their earring, selling out ditzy possible Dark Action Girl Shiro to the guards, and killing his own father, although the last part was because he thought he was protecting his sister. Then again he doubted that his dad could've really raped her so one wonders exactly what kind of person You was before being imprisoned.
Actually, his little sister, Minatsuki, killed their father. He even asks, upon her about to Back Stab him with her Branch Of Sin, if 'this is how you killed our father?'
After his confrontation with Minatsuki, he actually starts becoming happier/more content, and babies the hell out of his little sister, which is pretty strange considering that Minatsuki is an Ax-Crazy, Psycho for Hire, Combat PragmatistBlood Knight who is a very experienced killer and has been a Combat Sadomasochist for the last 5 to 10 years. However, this change seems to coincide with the fact that Minatsuki mellows out from outright sociopathic killer to just Sociopathic Heroine because of her interactions with Ganta and You. That said... if you threaten Minatsuki, You will probably try to beat you up, but if you piss Minatsuki off, she will probably tear your head off. Literally. Nuff said.
Also from Death Note, Villain Protagonist Light. He presents himself as positive, thoughtful, and cheerful to the world, but at the beginning of the series he's so bored and empty inside that a quest for a new world and godhood seems like a great idea; and of course, as the series goes on, that smile hides an increasingly unstable and cruel personality.
D.Gray-Man: Lavi. Or perhaps in this case we should say Bookman Junior, since "Lavi" is just another one of his aliases. (He started changing once he started to get into his new role more and came close to his teammates though.) But what was the actual person like before joining the Order? During the fight with Road, Lavi gets flashbacks- what we see is a jaded and cynical young man who has lost all the faith in humanity, who doesn't give a damn about anything. He socializes with people only because it helps him to get information more easily ("Let's be frivolous and friendly like always"), face all smiles, not being a bit like keet we know he is in the series. It's even mentioned in Reverse Novel 2 how his eyes seemed dead when he first came to Black Order.
But when you consider his past, is it any wonder he turned out like that? After all, growing up while recording wars and seeing bloodshed wherever you go isn't exactly the happiest childhood there is. To sum it up: Bookman Junior is Empty, perhaps a bit milder version since he's not totally soulless, maybe a bit hollow, yes, but not empty. After becoming Lavi, his fake smiles have started slowly turn into genuine ones.
"1 year...2 years...The time within the order passed on. I soon came to the point where I didn't know whether my smiling face was a lie or not."
Some interpretations of Allen Walker's character. Allen tends to smile and throw out optimistic and enthusiastic support at the drop of a hat when around his friends, but whenever he's alone he tends to be either beating himself up, wallowing in some deserved self-pity, or worrying about his entire freaking existence. He's even called out on it by his mentor. When his foster father died, he went into trauma, and had to be taken care of for months. When he finally snapped out of it, he adopted his father's personality. Pre-trauma, he was rude and foul-mouthed. Now, he's somewhat repressed. He hid behind false smiles, and would sometimes claim to be fine when he wasn't. In a message, his master told him to stop wearing the mask of his father, and be his own person if he hasn't lost hope.
Sheril Kamelot qualifies as the unstable Stepford Smiler. He's always smiling but after the Noah kidnap Bookman and Lavi he goes absolutely nuts trying to figure out Road's relationship with the 14th.
Dragon Ball Z: Future Trunks. In his first appearance, he's friendly to the Z Warriors and is polite to a fault with them. However, he comes from a horrible future where every main character is dead. Made worse because his father Vegeta apparently ignored him, though at the end he leaves at peace.
Bulma: Hey, I'm just gonna throw this out there, but you're really cute. Trunks: Well, my mother always said I was a cute kid... Bulma: Oh, a momma's boy, huh? I'll beyourmommy. Trunks: (internally) AAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!!
Part of the Dragon Ball Z fandom sees Gohan as this, post-Cell Saga. Considering what the poor guy wentthrough, it's not hard to understand why.
Durarara!!: Kida is not naturally that cheerful; it's a front he puts up for Mikado's sake.
Yuki Sohma from Fruits Basket is so terrified of being rejected by his peers that he uses his numerous skills (martial arts, good looks, charisma, etc.) to hide the inferiority complex that Parental Abandonment and Akito's Mind Rape gave him.
Shou Tucker appears to be a kind, gentle man, but in reality is insane enough to transmute his daughter and his pet dog into a chimera.
Selim Bradley from the manga and Brotherhood anime is a sweet little boy. He's actually Pride, the eldest, and arguably most sociopathic, of the homunculi.
The Dwarf in the Flask - later known as Father - needed to be this to fool Slave #23 - later known as Van Hohenheim - into going along with his plans.
King Bradley is this; he appears to be a kindly family man but is actually a homunculus. He really fits the trope to a T because when he was human he spent his entire life from infancy on being trained to take on the role of "President Bradley." He was manufactured like a stepford wife.
Chichiri from Fushigi Yuugi has a literal mask that helps him pull off the goofy, care-free smile; it's a significant plot-point when he takes it off to relay his tragic backstory.
Game X Rush has smiling, laughing, charming Yuuki, who's hiding a treasure-trove of Capital-I-Issues in his head. Memori reflects that "Even a face laced with a tiny bit of pain... is a thousand times better than seeing that mask." It's implied that a large part of Yuuki's impetuous to stay around Memori is that Memori can discern the real smiles from Yuki just being a Sad Clown.
Andrea Cavalcanti/Benedetto in Gankutsuou is an effortlessly charming man who smiles sweetly at all times who happens to also be a crazy wild-eyed rapist with daddy issues.
Tohma Seguchi of Gravitation initially seems to be a polite, mild-mannered man until he shows other characters just how far he's willing to go to protect his brother-in-law Eiri, which includes pushing a man in front of a car and making Eiri break up with Shuichi by threatening to hurt Shuichi if he didn't do so. All while smiling calmly. He stops smiling for more than one panel only when he's in full-on Yandere mode.
In the manga sequel, Tohma seems to have shifted from mostly Empty to Unstable. He holds an axe over a comatose Shuichi, threatening to kill him if he doesn't wake up. He does this while still maintaining a huge smile. He's completely Unstable in the 10th volume when he comes face to face with Yoshiki Kitazawa.
Tohma's younger cousin, Suguru Fujisaki, is also a Stepford Smiler early on. At least until Shuichi's lazy work schedule gets to him and Suguru ends up panicking about how the band will ever finish its next album instead of worrying about maintaining a smiling appearance. He still has moments where he forces a smile, like when Tohma confronts Suguru in his office about who will be paying for the destruction of NG.
Itsuki Koizumi from Haruhi Suzumiya always keeps a constant smile and a cheerful attitude, regardless of how much is Haruhi casting upon the Universe. Granted, he's still mostly sane and calm, but his Stepfordness always creeps Kyon out to no end. But when his smile disappears... you better start fearing the worst! He actually expresses an open jealousy towards Mikuru and all the other members of the SOS brigade, because he has to keep a facade of his true personality to fit Haruhi's expectations of his, while the rest do not (aside of Mikuru's Obfuscating Helplessness). At points in the novel it's made pretty clear that Itsuki's finding it difficult to keep smiling, such as in volume 9 he's been unable to sleep well for a lengthy period of time due to constant closed space activity, appearing exhausted and using a rather forced version of the usual grin, which falls off if he isn't concentrating. What is he really thinking? Who knows, it's the one thing he doesn't really talk about. Probably.
To add fire to the fuel, in the eleventh novel, he cheerfully prepares to fight (and probably kill, it wasn't made clear afterwards) the novel's Big Bad. Note the word cheerfully.
The titular Hayate the Combat Butler. Recent chapters/episodes have implied that his kindness and decency are coping mechanisms he developed to cope with the harsh, nay, downright vicious realities of his life rather than his fundamental personality.
He's also a deconstruction of shonen heroes, and possible a rare type of psychopath with fascinatingly Protagonist-Centered Morality (i.e. "This serial killer helped us learn a lot by fighting us, so I'll show him mercy in good, shonen-hero style, even when you point out that he will go kill more young women and eat them if set free. Because he helped us.") It puts the manga-trained reader into the weirdest headspace after a while.
Well his best friend Killua has killed possibly hundreds of people in the past. Gon himself, in the life of a Hunter, will probably have to kill someone at some point. Its hard to condemn someone for killing when doing so will make you a hypocrite.
During the Ant Arc he kind of snaps. No past-pain comes into it, though. Just his failure that got someone important to him killed and his determination to be revenged.
And Hisoka is a rather crazy Monster Clown and frequently wears a smile on his face.
Mikael from I'm Gonna Be an Angel! . He usually tries to act all calm, cool and composed - like in the first 13 episodes of the series - smiling gently when talking to Noelle and being all nice and polite when dealing with her family. But when his teacher, Raphael showed up, it turned out that Mikael, when indoors, is just a very serious, uptight and insecure boy. And then he went all insane and evil when things didn't go as he planned and ended up just pathetically broken. Fortunately, he got better and returned to his uptight and highly anxious self.
Soubi of Loveless is a pretty extreme Empty. He seems happy and cheerful, but is actually an Extreme Doormat whose been essentially conditioned into only existing to obey his master.
Kaito Kuroba/Kaitou KID from Magic Kaito thrives on his use of "Poker Face," an important skill taught to him by his father when he was young. Kaito's poker face (both as Kaito and KID) is pretty much always some variation of a smirk, and he rarely lets it drop even when things go wrong. Let's put it this way: if Kaitou KID ever stops smiling, everything has Gone Horribly Wrong.
Shouma: "I've always thought I was a more pathetic and helpless person than I seem to be on the outside."
Kumagawa Misogi of Medaka Box manages to oscillate between all three types somehow. From his first appearance it is obvious, that his smiling and cheerful appearance not so much hides his evil and insanity, as is cultivated to disturb his opponents even more. Kumagawa's former doctor believed that he really had no emotions at all, until at one point he got actually enraged to the point of losing control. And it is quite believable (not thanks to his Motive Rant, as he's a compulsive liar, but thanks to the track record of this manga so far and his later Heel–Face Turn), that he is also extremely embittered and twisted.
This applies to all Minuses in Medaka Box. As Kumagawa put it:
Even if things don't go the way you want them to. Even if you lose. Even if you don't win. Even if you look like an idiot. Even if you're walked on. Even if you're kicked. Even if you're sad. Even if you're bitter. Even if you're tired of it all. Even if it hurts. Even if it's hard. Even if you're weak. Even if you aren't right. Even if you're humble. Through all of that, we Minus always laugh.
Duo Maxwell from Mobile Suit Gundam Wing who seems to be upbeat (and snarky), but really is traumatized by the war and losing many people who were close to him.
Duo is the most cynical pilot by some way, and also the only one who doesn't seem to have something childlike about him, although he's the one who seems the most childish on first meeting. He hasn't been sheltered, trained, or broken, just hurt and rolled with the punches; he's almost certainly the most stable of the five of them. Cynics are hard to surprise, after all. So...questionable, despite Duo Torture being a major hobby of the fandom.
Despite being weird, the English dub that made him sound forty wasn't the worst casting choice ever.
Muruta Azrael of Mobile Suit Gundam SEED. While his evil is public knowledge, his enthusiastic personality and dark sense of humour are a ruse to cover just how unstable he is. When he finally slips up in the penultimate episode, the results are nothing short of horrific.
In fact, Johan's own ability to project a happy, kind, innocent and childlike demeanor worthy of a cherub, other than for purposes of manipulation and as a cover for his true sociopathic nature, doubles as a mask for covering up his dark past, spent in the orphanages of the Communist bloc.
Also, Johan's muscle and pawn, Roberto, harbors an extremely creepy Stepford Smile, as seen here◊. Let's just say that it's a common trait amongst Kinderheim alumni...
Naruto himself. He smiles near-constantly, which, during his childhood, was to mask the sadness of his lonely life. Those who are closest to him are able to recognize when his smile isn't genuine (which is, surprising enough considering his personality, much of the time). He will tend to mask his pain with a smile, such as when he promises Sakura to bring back Sasuke for her before and after the retrieval mission. He even admitted to Hinata that the smile is a mask, and Pain put him on the ropes by breaking him down with words, getting him to second guess his own actions. Before the Chunin exams finals, he admitted that he viewed himself as a loser and tried to act cool and brash to cope with this.
Make no mistake that Naruto is an optimistic, lively person by nature, but given how little reason he had to smile in his childhood, it's small wonder not all of it is totally genuine. If he didn't have the zest for life which he generally displays, everything Naruto had to deal with would have probably driven him mad. As he slowly gains recognition and respect, the smiles appear less but in turn become more genuine.
From the same series, Sai. As part of his indoctrination into Danzo's forces, all of his emotions were stripped away until he was emotionally empty. When assigned to Naruto's team he tried to teach himself how to smile in order to pass himself off as normal, but Naruto easily saw through it. He still has trouble understanding and feeling emotions, but he's getting better... He will never be normal, but who wants normal?
Iruka was a mild example during his childhood (after his parents' deaths).
Mizuki shows signs of being Unstable, especially in his Monster of the Week arc. Even as a child he was shown to have virtually zero empathy for Iruka's suffering.
More specifically, he tells Iruka that he acted supportive, hoping the positive attention Iruka recieved would rub off on him. His girlfriend believes he was once genuinely kind-hearted, before the Envy he had slowly made him cruel until the good was gone (it probably didn't help that suspicions on him killing a teammate on a mission led to him being distrusted and denied a promotion, which made him more resentful). Whichever is true, his kind smiles are certainly a lie in the present.
Itachi's gentle and loving, if somewhat distant, attitude in Sasuke's flashbacks before the massacre count as Unstable. Can probably be considered Depressed once we learn the truth about him.
Jiraiya puts himself down a lot despite acting goofily perverted and smiling a lot. Even in his last moments he believes that his life was a complete waste and that he was a loser. His only goal in life was to have an awesome death. He managed it.
Kakashi too, who smiles when he tells Sasuke everyone he loves has already been killed, and generally acts rather carefree, but is actually filled with self-loathing and feelings of guilt over his teammates' deaths.
Shion of No. 6 is hinted at being this. He smiles constantly and is generally very cheerful and sweet but he's also hinted to have a suppressed Yandere side to him. Beware the Nice Ones indeed.
Monkey D. Luffy is thought of this by a good portion of the One Piece fandom because of his relentlessoptimism considering all of the terrible things that have happened to him in the past. But the main reading is still that he's just that stupid.
A recent case has the three princes of the Ryuugu kingdom (Fukaboshi, Ryuuboshi, and Mamboshi). The three are seen smiling and, in the last two's case, singing and dancing in a last attempt to keep their little sister Shirahoshi calm... after all four of them just witnessed their mother Otohime being shot and dying.
Also thought to be this are the Boa sisters and some of the other slaves of the Celestial Dragons.
Brook is definitely one of these when we first meet him. Prior to encountering the Straw Hats, he's been trapped on his broken-rudder ship, drifting around the foggy seas for 50 years after he and his crew died. And five years ago he lost his shadow to Gecko Moria, meaning he can no longer go into the sunlight without disintergrating. And it's implied in flashbacks that the isloation did something to his head. Now he acts like an Upper-Class Twit with poor manners. As he energetically summerizes his existence, Sanji asks, "Why the hell are you so cheerful? Your life sucks!"
Tamaki Suoh from Ouran High School Host Club is this. He has to live with the knowledge that he'll never be the heir to the Suoh dynasty, that his grandmother despises him simply because of birth circumstances (his father married a lower-status French woman instead of the arranged girl) and that he may never amount to anything famous. Until the last two episodes of the anime, he covers it up remarkably well, mostly by holding ostentatious events with the host club. We also hear about it during Kyouya's backstory, when he explains how he and Tamaki became friends and started the host club.
Break is also one. Then again, having the family that you tried to save end up dying even at the cost of losing your eye is enough to shock someone into faking smiles forever. He never lets the smile fall except when he's close to his breaking point. Like many of the characters in this series, his smiles seem to become more and more genuine as the story progresses.
Jack is a combination of all three types. He is described by many characters as appearing "empty" inside even before Lacie's death. Needless to say, he's quite the complex character in a series known for complex characters.
Levi, one of the past Glens, qualifies for all three types. It's pretty hard to keep smiling like that when you're rotting alive painfully and one of the people you cared about ended up being dragged to the Abyss to be Deader Than Dead, but somehow, he managed. His "friendly advice" to Jack reveals him to be somewhat Unstable as well.
Leo has become this way as well after Elliot's death. He's equal parts B and C, having essentially Stopped Caring, and Oz even comments, "You don't want to destroy anything. You're just trying to destroy yourself."
Okita "I smile constantly, love candy, play with the neighborhood kids, am pretty much a male House Wife to Hijikata, and am pretty much the most terrifying swordsman in all of Japan" Souji. Okita "I've been killing since I was eight and don't appear to actually regret it" Souji. That man's a TEXTBOOK Stepford Smiler!
This may be why his Gintama counterpart keeps trying to murder Hijikata.
Shuusuke Fuji of The Prince of Tennis was a bit like this due to the problems with his younger brother Yuuta that caused him to bear a huge guilt. He starts acting more genuinely cheerful when he and Yuuta patch their relationship.
Isabella in The Promised Neverland is as sweet and loving a mother as one can have. At least she does her best to give that impression to the children in the People Farm she's managing for her demonic patrons. Considering it's implied that her own life is at stake, she has some reasons to keep a perfect act, but she still seems to enjoy herself a bit too much in this job.
Natsuki Subaru from "Re:Zero" starts to become one of these as the series goes on. Possibly even started out as one from the beginning. As the series goes on it becomes clear that his happy-go-lucky attitude is covering up severe self-doubt and depression from trying to save his friends only to see them die over and over again and regret with how he spent his old life on Earth.
The villainous Seta Sojirou, The Dragon of Rurouni Kenshin's best known story arc, also fits here — a cheerful young person who's capable of incredible violence because he has no soul or conscience behind the mask. (Or at least that's what he thinks.) He actually started doing this because it stopped his family from abusing him if he didn't appear bothered by it.
As for his reasons? Soujiro was constantly beaten as a child by his family due to being an illegitimate child, and learned to smile as a defense mechanism to get them to stop. He did this so much that he subconsciously repressed all his emotions, projecting nothing but a pleasant smile, even as he kills people. As a swordsman, this makes him virtually impossible to read and predict, and in addition to his speed and natural talent with the sword, this makes him a very dangerous opponent. This all changes when Soujiro battles Kenshin for the second time: Kenshin's worldview clashes with Soujiro's, irritating him to the point where his emotions finally return to the surface, causing him to break down in confusion. This state of emotional turmoil is what allows Kenshin to finally defeat him.
And again, Okita Souji, in the brief times he appears to interact with Kenshin and Saito Hajime. This is the popular portrayal of him in Japanese fiction in general, and it has been stated by one of his descendants that he did indeed smile frequently. (This is probably because Sojirou's character was based on Okita Souji.)
On that note, Kenshin himself. He hides the guilt and shame over his checkered past (and hides his temper) behind a cheerful and laid-back persona. Common for Walking the EarthRetired Badass types. The key difference is that the smile is not meant to be shallow; Kenshin really wants the smile to be real but keeps having to deal with his past, placing him somewhere between this trope and Becoming the Mask.
Cho Hakkai from Saiyuki qualifies, as he smiles constantly even while fighting, despite his angsty past which includes but is not limited to killing over 1000 demons to rescue his lover Kanan (who is really his twin sister), then watching Kanan kill herself with his sword because she was raped and impregnated by a demon, then to top it off, becoming a demon himself. Whew. His companions note that whenever he actually does stop smiling and looks angry, it's extremely scary.
Manages to overlap with Slasher Smile without changing his expression. Creeeeepy.
Certain chapters and episodes of Seitokai no Ichizon imply that Ken's happy-go-lucky persona is a defence mechanism to keep him from a severe emotional trauma from back in junior high, and not his actual personality.
Seraph of the End: Both Yuu and Mika. In the novels its stated that Mika deals with the desolate situation involving his real family that abused him and his adoptive family of other orphans and Yuu being subjugated by vampires by smiling cheerfully. After his turning into a vampire he becomes more of a Perpetual Frowner and only seems to smile around Yuu.
Yuu's cheerful side and ability to never give up even when faced with impossible odds (even Ferid notes it in the novel) is something he either naturally had or developed in the orphanage and in the vampire city under the influence of the kids and Mika. So in a few recent chapters it is both a facade he uses to cheer up his friends and lessen their worries and a manifestation of his own determination and positivity (he is aware of how grave his situation is, but he is determined to fight to the end with everything he’s got to change it).
It's been implied in the Catastrophe light novels that Shinya's smile developed as a defense mechanism that allowed him to survive when those that got angry or shut down didn't.
Sgt. Frog: Kogoro, who never stops smiling. He's far less dangerous than the usual example though.
His sister, Lavie, is the only one who can read his emotions.
It's been implied at this point that Takuto Tsunashi of Star Driver, up to this point a typical kind, cheerful, badass Idiot Hero with a bit of Cloud Cuckoo Lander mixed in, at the very least has some Hidden Depths that are... well, not nice, at the very least (and at the most, this is all a facade and he's completely SCREWED UP) if these implications are to be believed. Knowing Yoji Enokido, these are probably going to be fully explored by the time the series is over.
Kotetsu from Tiger & Bunny is implied to hide feelings of worthlessness and unhappiness with his life under his Hot-Blooded demeanor. Word of God supports this, stating that Kotetsu exaggerates his behavior as a means of putting people off of him (because he doesn't think he's worth worrying over), that he's never fully sorted out the emotional baggage anti-NEXT discrimination's saddled him with, and that he was on the verge of major depression at the beginning of the series.
Togainu no Chi: Keisuke has shades of this in Episode 5 where he smiles at Akira despite it being clear he is disturbed by what Akira had done. Naturally, Akira calls him out on it.
When asked whether he's curious about his past in Tokyo Ghoul Sasaki cheerfully informs Arima that he's "very happy" with his current life. That he touches his chin while smiling (it means he's lying) reveals he's not nearly as happy as he'd like the world to believe. In general, he masks his massive emotional and psychological issues behind a cheerful smile, terrible puns, and a gentle attitude. It's later implied further that he's absolutely terrified of Arima.
In Tokyo Mew Mew, Masaya Aoyama is revealed to be one of these, keeping up his "perfect" mask to hide the fact that he hates all of humanity.
Trigun's Vash the Stampede combines this with Obfuscating Stupidity to create Crouching Moron, Hidden Badass so that nobody realizes just how dangerous he is, or what horrific memories and experiences he lives with every day. In the whole anime series, only Nicholas D. Wolfwood is able to see right through Vash's act - in his first episode, Nicholas shares some food with a pair of hungry children while Vash watches, then tells Vash that he looks good with a real smile on his face, not the fake one he's been showing.
Wolfwood has the advantage of inside information. The old Professor among the Last Unfallen humans, who's known him for decades, can also read through this kind of thing, although Vash doesn't try it so much with him. Stuff like bickering childishly with Wolfwood apparently reads as 'real' to both these parties.
Manga!Millie, who's dumbfounded by this, eventually compares Vash's fake smile with the grey colour you obtain when you mix paints of all colours.
That's in Maximum #39 'Colorless Emotions,' the first time Vash reflexively uses the angel arm to block a bullet in a relatively casual gunfight. (Read: the people trying to kill him aren't plot-related characters.) Meryl cracks up briefly at seeing it after the trauma she had last time he used it, and the townspeople conclude he's a demon and start ''stoning'' him. Wolfwood snarls at them. Crowning Moment Of Heartbreaking:
Millie: Vash! I don't understand. What's happening? What's wrong? When it's as hard as this...you can still keep smiling like that?!
Vash: [Beat panel, still smiling.] What other expression should I make? I don't know anymore.
Nicholas D. Wolfwood himself thrives on this trope, although it's clear almost from the beginning he is a dark, conflicted, and potentially angsty character.
And let's not forget that in the manga, young Knives managed to hide his feelings of agony and murderous hatred from Vash and Rem by having fun with them just as usual, making crepes while smiling and laughing, claiming to go all 'business as usual' because he had to go on with his life etc.
Even before the series hit rough territory in the Acid Tokyo arc, in Outo Country Kurogane had blatantly told Fay he knew he was hiding something. During the Celes arc we find out just what he had been hiding. We learn about his past and that his real name isn't Fay, but Yuui; Fay was the name of his identical twin who sacrificed himself to let Yuui live... but Yuui thought he killed his brother. That's not all, either. Fay has other dark secrets we learn about. No wonder he tried to hide behind a smile.
Kei Yuzuki from the second Vampire Princess Miyu OAV. He hides his boredom with life in general, his feelings of inferiority towards his more academically-successful siblings and his apathy towards his Education Mama's constant pressure under a polite, soft-spoken, beautiful princely mask.
Also Lemures from the third OAV, who appears to be a helpful Dandy in his human form, but is soon revealed to be scary and a Yanderefor his old friend Larva, whom he has kidnapped and imprisoned in hopes to free him from Miyu.
Amasawa of The Weatherman Is My Lover, as in spite of his outgoing and eccentric personality he secretly never got over the guilt of his parents' deaths and lives a really isolated existence as a result.
Ryou Bakura of Yu-Gi-Oh! is one of these. When he first appeared in the manga, the Millennium Ring was giving him chest pains, but he covered it up so people wouldn't find him strange. He overall acts incredibly cheerful most of the time, but he also hides his problems, which is pretty bad for the protagonists. Yami Bakura, however, takes advantage of this, as he often pretends to be the normal Bakura so he can carry out his evil plans on the quiet, meaning he does this on purpose.
He ambiguously stops existing at some point in the third season for no particular reason. His friends apparently do not care. Or notice. Or something. He kept turning into the crazy guy so they couldn't trust him, and the pharaoh decided killing him was acceptable given the threat to the world, so maybe he committed soul suicide. Or just retired to his soul room forever.
Judai/Jaden Yuki of Yu-Gi-Oh! GX pretends to be goofy and friendly, doing many shenanigans through the series. Until you find out halfway the third season that he used to have a card with a duel monster spirit that would send people into comas if they upset him when he was a little kid and then he sent the said card to space to irradiate it with cosmic energy to make it better, but instead it got attacked by the Light of Destruction, making the monster feel pain and due to his connection with it the boy had nightmares until somehow they managed to suppress his memories, making him having amnesia. The US dub cut 80% of said backstory.
To make things worse, during all the previous seasons in the Japanese version there is a strong criticism on hierarchy and status quo, and the said character hates it with a burning passion effectively doing whatever he can against that to the point that during the season 4 the rank system was mostly moot.
Yuya Sakaki of Yu Gi Oh ARCV is the depressed variety. His father, Yusho, taught him to laugh whenever he felt sad. When Yusho disappeared one day, Yuya started to make a fool of himself to cope with the disappearance.
Krista Lenz is the sweetest, kindest girl anyone has ever met and referred to as the "Goddess" of the 104th as a result. She's actually deeply troubled, with a dangerous martyr complex and a need to be seen as virtuous and kind regardless of the risk it places her or others in. She's getting better.
Reiner Braun is a surrogate big brother to the others, always trying to offer reassurances or awkward jokes even in the worst situations. He's actually being driven slowlyinsane with guilt over his crimes as The Mole.
Birdy the Mighty: Decode has a number of characters who show signs of being this.
Satyajit Shyamalan, the first season's villain, presents as a friendly, sweet-looking, successful TV executive, but is in reality a Social Darwinist who thinks nothing of resorting to murder to achieve his goals, plots to use a super weapon to wipe out most of humanity, and is implied to be desperately lonely and have deep psychological scars stemming from getting caught up in a terrorist attack that killed everyone around but him.
Outside of the ones that act fairly emotionless, many Contractors are like this in Darker Than Black. November 11 and Amber are a good illustration, as their cheerfulness seems to be inversely proportional to their sincerity.
Ayano, the core of her family after her mother Ayaka's death, Manic Pixie Dream Girl to Shintaro and Living Emotional Crutch to Kido, Kano and Seto (especially Kano - see below) spent the last year or so of her life Depressed; a few months after her mother's death, she found evidence that the old Medusa Legend actually had some basis in reality, and that her father had been possessed by the most evil snake of them all - the Wide-Open Eyes Snake. She also discovers that his plans involve the deaths of her surrogate younger siblings and half her social circle, so she's understandably downtrodden. Despite keeping up her Manic Pixie Dream Girl persona around her friends, we are shown that beneath that, she is emotionally exhausted, and desperately searching for a way to save her siblings and friends. The solution turns out to be her own suicide.
Kano. After his self-sacrificial/abusive relationship with his mother, Kano latched onto Ayaka, and later Ayano, as female role models/mother figures, and when they were taken away one by one, his mental health took a serious hit. The fifth novel and anime also reveal that he was involved with Ayano's investigation of her father, and was standing right there when she jumped off the roof of the school. Following this, the Wide-Open Eyes Snake (who was also present) essentially told him that he would murder the little family he has left if he tells anyone, and that their fate is unavoidable. By the present day, Kano finds it so hard to be happy that he actually has to use his powers to just giggle. When all of this inevitably comes out, he almost unleashes hell on his brother Seto]].
Before her death, Ayaka is implied to have been Depressed. She is always referred to as the glue holding the Tateyama family together, yet her journal reveals that she spent every day fervently researching the Medusa Legend, for fear of what would eventually become of her adoptive children Kido, Kano and Seto - who had already been possessed by snakes.
Ene also counts; despite appearing as a 'shallow' (in Shintaro's opinion) program that is perpetually energetic, cheerful (and occasionally, deceitful), she is actually Takane's consciousness - after being killed, losing her body and wandering the cyber world for months with no direction. Oh, and her last memories before death were running to Haruka's side, after he was hospitalised because she ignored him when he was having an attack. When she discovers that Haruka died after that... it's not pretty.
Even though Momo tends to act like a text book Genki Girl the anime shows that she struggles quite a bit with her inferiority complex, plus the stress her powers and work as an Idol put on her.
Kare Kano: Both Arima and Yukino are Depressed. However, Yukino's version is played for laughs because hers stems from her own narcissism and desire to have people compliment her. Arima's, on the other hand, is played as drama as he tries to coverup his insecurities of that developed from being abused and abandoned by his parents and being considered the Black Sheep by the rest of his extended family.
This all culminates into a relationship between the two, with Arima thinking some...terrifyingly possessive thoughts about his girlfriend who is oblivious to the dark turn his thoughts took.
In Sands of Destruction, Morte becomes one for a bit after she realizes Kyrie was the Destruct, and she managed to drive him away (and thus lose her change at using his powers) by being too psychotic. She puts on a cheerful face for her shipmates, but the manga makes it clear she's heartbroken inside.