GB (a.k.a. 007) from Cyborg 009. His backstory changes in every animated continuity, and yet none of them are pretty.
Duo Maxwell from Gundam Wing tries to be the moodmaker for the Gundam Pilots, and is certainly the cheeriest and goofiest of the group. He's also got a particularly tragic past (orphan, lost his best friend to disease because only the rich people got the vaccine, bounced between foster homes before settling in a church with a caring priest and sister, only to lose them to a pointless battle) and refers to himself as "Shinigami", not boasting about his ability to kill enemy pilots but because the specter of Death seems to hover around him at all times.
A more literal example is Trowa Barton Triton Bloom, who works in a circus as a clown, but is as far from goofy as one can get.
There's a female one in Hana no Ko Lunlun. Sophia the Norwegian Circus Brat is embarrassed about working as a clown in her family's circus (despite having the talent to do so) and lies to Lunlun about actually being a Cute Monster Girl instead. With Lunlun's help she discovers that it's actually fun to make people laugh.
Rare female example: Anna Heart from Kaleido Star, a Bifauxnen artist from the Kaleido Stage who wants to be a comedian, but hides how deeply hurt she's been after her father Jack (a.k.a. "Baron Jack") left her and her mother Julia.
For that matter, Jack was one of these too. The reason why he left is that he was swindled by his manager, and left home because he couldn't face Julia and Anna as well as the crash of his dreams. And he was getting worse, evolving into a very embittered Stepford Snarker who told cruel jokes that mocked everyone (something he had never done in the past) until Anna got toconfronthim. Thankfully, they got better.
Izumi from Martian Successor Nadesico is a one-woman Hurricane of Puns. We never get the full story, but one episode reveals she only became like this after her fiance died in an accident. When she's forced to relive it she nearly turns suicidal. It wasn't even the only boyfriend she'd lost!
The eponymous character of Naruto ditches the joking clown for more of a mad jester routine, constantly insulting others, creating nicknames, and pulling annoying pranks. Much of this results from his insecurities after growing up as a reviled orphan, where the negative attention he got via his acts was still better than what he got by acting normal.
Iruka is implied to have been like this, having lost both his parents to the Nine-Tailed Fox. Unlike Naruto, he did not have to deal with being hated, but he acted the way he did to ease his loneliness. He reaches out to Naruto when he realizes they have this in common and becomes Naruto's first father figure.
Possibly Luffy as well, considering the things we've learned about his past in the last few years. After his complete and utter breakdown after Ace's death, it becomes devastatingly clear that underneath his cool, strong Idiot Hero exterior, there's a scared little boy who's terrified that he's too weak to protect his loved ones, and that he's generally nowhere near as innocent and carefree as he might want you to believe.
Brook practically runs on this trope, being a walking pun factory on the subject of his supernatural disfigurement. At inappropriate times? You bet. Internal monologue and a very blunt conversation with Franky have made it clear enough that he's not really all that happy with being a skeleton, though...and then he was put on display in a freak show cage for crowds to scream at in disgust. Then he met the "get people to like him" criteria in truly epic fashion. It's good to be the Soul King.
Luffy's brother Ace is a pretty jovial guy, much like Luffy. And just like Luffy, he's got a lot of hidden insecurities. He spent his entire life thinking that he didn't deserve to be born.
Break from Pandora Hearts teases everyone merciless, smiles constantly and has a ventriloquist (probably) act going with a doll he keeps on his shoulder. And is as utterly broken as if not more so than anyone else in the main cast.
Sasame flirts (heh) with this trope in the manga version of Prétear. He's constantly flirting with the main character and teasing the other knights, and at first it seems like he takes NOTHING seriously... but several scenes hint that he's not quite as much of a jokester as it seems. This particularly comes into play when he reveals he was in love with the Big Bad in the past, and you consider what happened to the anime version because of that...
His counterpart in X-Men: Evolution is Kurt. Who has two or three shades of this in the comic, actually.
And from X Factor we have Guido (a.k.a. Strong Guy). Cracking dumb jokes helps ease the physical pain from his mutant powers. To say nothing of the problems that come from just being a mutant.
Spider-Man himself is a rare example of a main character being the Sad Clown. At his healthiest, his notorious mid-fight quips are still as much about coping with how scary his life is as distracting his enemy.
Iron Man: Have you noticed the closer we get to uncomfortable truths, the more jokes per minute you make?
Iron Man himself verges on this at times, especially when Matt Fraction writes him.
In Marvel Zombies, Spidey drives the other zombies crazy with the constant, irritating jokes he keeps making. When told to cut it out, he informs them that he makes jokes to help himself forget that he's become a flesh-eating evil lunatic.
Mary-Jane Watson was one of these in her backstory - she kept up a constant Fun Personified party-animal persona to cover up for how miserable her home life was.
Mentioned in Watchmen throughout flashbacks of Eddie Blake (a.k.a. The Comedian). "But Doctor...I am Pagliacci!"
Plastic Man from DC Comics is often accused of this, denying it every time. Whether he is lying or not depends on your interpretation.
Dick Grayson has been retconned into this in his youth. He was the first Robin, and his history is largely unchanged: he was the same person cracking jokes and facing down villains and making terrible, terrible puns. However, his parents were killed in front of him, and his adoptive father figure has been transformed into always having been a brooding creature of darkness, so he was covering for something. As Nightwing, he's less of this, being relatively well-adjusted, all things considered.
If the memories of The Joker in The Killing Joke (wherein "Jack" loses his wife, the baby inside her and his face in one day) are to be believed, he plays this trope straight. The heartbreaking finale wherein both the Monster Clownand Batman hysterically laugh at the cruelty of their lives drives in just how deeply both these men have been hurt; Joker in particular must substitute laughter for tears, or the ponderous weight of his sadness would crush what little will to live there is left inside.
If you feel a bit uncomfortable feeling that much sympathy for him, the comic allows you to play the Multiple Choice Past card and go on hating him. Honestly, that's clever.
In Death Of The Family, Harley Quinn shows indications of this. She has tears marking her face, and she cracks jokes as she tries to survive being around Joker. Joker, once again, himself implies this about him.
Although he is a genuinely happy-go-lucky person, Morph has instances of this in both his Age of Apocalypse and Exiles incarnations, which results in teammates telling him to shut up and be serious for once. When he does, it tends to be heartbreaking.
His original incarnation in the X-Men TV series was like this too, and it was just as sad to see his real psyche.
In the Gargoyles spin-off Bad Guys, Fang is revealed to be a Sad Clown. He's just as shocked and horrified as everyone else to find out Tasha hung herself. He just dealt with it by making an inappropriate light bulb joke.
Deadpool is often written like this. In the first arc of Cable & Deadpool, Cable asks why he's helping the villain's Assimilation Plot - it could be the first step to world peace, but falling in line and giving up his right to be different isn't Deadpool's style. Deadpool replies that all his crazy opinions just cover up the fact that he doesn't have anything.
He actually lampshades this trope in his arc in X-Men Origins, when he is telling a screenwriter about his less-than-ideal home life.
Deadpool: When you're confronted with a horrible situation, there are only two reactions that make sense: laughter or tears. And laughter, after all, is nature's anesthesia. Tears hurt too much.
Yorick from Y: The Last Man continues to crack lame jokes despite being the only male survivor of the Gendercide, to the frustration of his traveling companions.
In Red Hood and the Outlaws Arsenal tries to chat and make jokes whenever he can, but it's fairly obvious that he's practically dead on the inside. He admits this in Issue 5 to Starfire, saying he believes that as a team the three of them could help each other.
Christ, the entire team is dead on the inside and trying to cover it up.
Scientist Paul Beaumont in He Who Gets Slapped became a clown after his patron stole his work and wife. As a clown, Beaumont falls in love with another performer who is in love with someone else.
Jerry Lewis made The Day the Clown Cried, about a depressed, formerly great German circus clown during the Holocaust. Shooting on the film was completed, but it was never finished or released, due to behind-the-scenes disputes. It was locked in the vault, and has since become a Hollywood legend.
The 1930 German film The Blue Angel uses this trope for dramatic effect, as the main character's loss of dignity, fall in society and descent into madness are punctuated by his donning of a clown costume. His first performance in full clown make-up, where he is continuously debased and forced to crow like a cock, is the climax of the movie and shows just how pathetic the once proud man has become.
The clown in The Illusionist who drinks and listens to happy circus music. He did try to kill himself at one point, but Alice unknowingly stopped him.
Suggested as the In-Character motivation of Giacomo the Jester (Well... Hubert Hawkins' interpretation of Giacomo anyway) in The Court Jester, via the song The Maladjusted Jester. In brief, he was a morose child who didn't laugh much, to the concern and frustration of his parents. They consulted a witch who foresaw his talent for performance and comedy, much to the bafflement of everyone.
Kung Fu Panda had Po, a giant panda who initially had a well hidden self-loathing so profound that it made his heroes' initial disdain for him feel positively kind.
Chuckles the Clown in Toy Story 3 became this after Lotso took over Sunnyside Daycare, though he starts smiling a little again during the ending credits.
A Hard Day's Night had a praised sequence in which Ringo Starr had an affecting performance as a sad clown, but Ringo was not so much acting as he was hung over and so his miserable air was real.
The Pagliaccio joke:
A man goes to a doctor, claiming he's depressed. He feels as if the world doesn't care about his problems, as if he's the pole the universe pisses on. The doctor ponders the man's problems, unsure of what to do, until suddenly he remembers: "The circus is in town, and Pagliaccio the clown is there! Why don't you go see his show, I'm sure that'll cheer you up." The man breaks down crying and sobs "But I am Pagliaccio the clown!!"
Ephraim Kishon uses the joke in a different context. His joke starts the same way, but at the end the patient says instead: "Doctor, I've been at the circus, I've seen Pagliaccio. He wasn't funny at all. He was the unfunniest clown I've ever seen." The doctor breaks down: "But mister... I'm Pagliaccio!"
Marco from Animorphs. It's stated many times that joking is the only way he can deal with the difficult and dangerous situations he's constantly put in, and the fact that his mother is Visser One's host. Differs from the norm in actually being hilarious.
Another K. A. Applegate example, Christopher from Everworld is almost an Expy of Marco, but with a darker personality, being a budding alcoholic and purposefully making racial or gender-based comments to offend people. And he's not funny.
Silk from the Belgariad openly admits at one point that he makes jokes because the alternative is to break down crying.
In a rare moment of honesty, he stated that one of the reasons for his sadness was that, being perhaps the greatest spy ever, he had so many multilayered cover-identities that he had lost himself somewhere under them.
Clone commandoR C- 8015, 'Fi', starts out merely with wisecracks. Over time, however, his mood darkens, but he continued to amuse his squad mates with jokes. His tend to be a touch morbid, but very funny.
His Deadpan Snarker contrast is his replacement, Corr, who copes with bad situations with acid sarcasm.
In the X-Wing Series, we get Ton Phanan, though we don't really see the "sad" part until we get to Iron Fist.
Bryan Stark, the main character in the teen series DRAMA!, is a mild example of this.
In Robert A. Heinlein's Stranger in a Strange Land, Martian-raised Mike has difficulty understanding humor, until he draws the conclusion that all humor serves basically this purpose: "They laugh because they hurt so much. Because it's the only thing that will make them stop hurting."
Members of the Fools' Guild in Discworld often lapse into this, sometimes forgetting if they're supposed to be "happy on the outside or crying on the inside, or the other way 'round." This is understandable, since the Fools generally don't lead pleasant lives. Indeed, it is common knowledge on the Disc that Fools simply cannot be actually funny ("if it was funny, a clown wouldn't be doing it"), and clowns are regarded as inherently pitiful or scary, rather than comedic.
Exemplifying this is the fact that the guildhall is often mistaken for that of the Assassins, which is actually the light, airy building next door.
According to Word of God, the Fools' Guildhall began as a monastery for a particularly sombre group of monks, and the founder of the Fools' Guild was shaped in his philosophy towards comedy by the fact that A: he was honestly nowhere near as funny as he believed himself to be, B: a mindset that convinced him that jokes and humor were Serious Business and should be treated with great dignity and respect, and C: the very pronounced trait of Discwolders, especially those in Ankh-Morpork, to be realistic and literal-minded to the point of being deliberately obtuse, which doesn't make joking a very easy matter. The result is that generations of Fools have had their emotions crushed and any actual knack for humor (not to mention desire to make people laugh) stamped out of them.
Leo from The Heroes of Olympus. He jokes to deal with the loss of his mom. And they're actually funny, unlike most Sad Clowns.
Even worse, Leo has been pushed from foster home to foster home, due to people seeing him as a "demon child" (he is the first son of Hephaestus who can control fire in over 300 years. And the one before him started The Great Fire of London.)
Nick Sagan's Idlewild has Mercutio, who describes his coping methods as "Humor? That's my lizard tail. You can look at that while I run away."
In The Wide Window, the characters patronize a rather miserably awful restaurant called The Anxious Clown. Guess what all the waiters are dressed up as.
Odd Thomas, in the series of novels of the same name by Dean Koontz, a First-Person Smartass who notes early on that he will be keeping the tone of the books light, otherwise what he has to say would be too painful to tell, and in-story (in other words, in the actual situations) he covers up his sadness, fears, etc. with a healthy dose of snarking or silliness.
Doctor Who: The Doctor, especially in the new series. In the words of Eleventh Doctor actor Matt Smith, "That’s what interests me about The Doctor because, actually, look at the blood on the man’s hands. 900 years, countless very selfish choices, and he’s literally blown planets up. His own race, you know, that’s all on his hands. Which is why I think he has to make silly jokes and wear a fez. Because if he didn’t, he’d hang himself."
Friends: Chandler is a rare example from a comedy series. This was lampshaded quite early on, when Phoebe's psychiatrist boyfriend, as part of his schtick of alienating everyone by pointing out uncomfortable truths, said, "I'd hate to be there when the laughter stops." Chandler is actually scarred from his thoughtless parents and neglected upbringing when it was implied he was used as a pawn in their divorce. He has serious self-esteem issues because of it and it's only through his relationship with the other friends (particularly his girlfriend Monica), that he gains confidence.
Generation Kill: Ray Person, although his insecurity isn't the only thing fueling his hilarious, uber-offensive humor - it's also the fact that he's generally on almost no sleep and on a near-permanent caffeine high thanks to his ever-present bottle of Ripped Fuel. Averted in the finale (and, after the war, in real life) when extra interviews show the real Ray as a generally quiet, yet outspoken man.
How I Met Your Mother: Barney Stinson, whose constant self-aggrandizing wisecracking, over-the-top stunts (everything from magic tricks to kidnapping his friends), and ridiculous womanizing antics are a mask to hide how utterly insecure and self-loathing he is inside, and how desperately dependent on his friends he is, by trying to make himself look like a loose cannon who is too awesome and confident to need anyone.
Keen Eddie: Throughout most of the show, Monty makes several ill-timed and insensitive jokes, warranting Eddie to suggest that he has no soul, but after getting snapped at for joking in the face of Eddie's life-or-death situation, Monty admits that he uses humor to cope with situations that worry or frighten him, siting how he laughed all through his uncle's funeral.
LOST: Hugo is usually seen as a jovial, Plucky Comic Relief kind of a character, especially in the first season. In his flashbacks, however, he is portrayed in a much more serious manner and develops into one of the most Genre Savvy and solution-oriented survivors.
M*A*S*H: Hawkeye Pierce was once accused of this. He was quite offended, and spent the rest of the episode refusing to joke.
The Mentalist: Patrick Jane fits perfectly. He hardly ever STOPS smiling no matter what, but the man is ridiculously full of self-loathing and guilt for his role in the death of his family. He can't sleep at night, he still wears his wedding ring eight years after the death of his wife, and his only home besides the CBI headquarters is his broken down old house where his mattress is right up next to the smiley face on the wall that is Red John's signature.
Not Going Out: Lee in this Britcom has been accused of being one of these by other characters, to the point of being cajoled into seeing a therapist.
Psych: Shawn. He admits that he relies on jokes and inappropriate humor to defuse tough situations, and when his funny breaks in "An Evening With Mr. Yang", Gus takes up his Sad Clown mantle to help him stay calm.
Revolution: Aaron Pittman, played by Zak Orth, is the comic relief of the show. He was once the Google CEO, he had a beautiful wife, lots of money, and simply had it all...until the blackout. Then he pretty much got left with nothing, and while his wife didn't leave him, he ended their marriage and struck off on his own because he realized that he wasn't going to be able to protect her ("Sex and Drugs"). He has a lot to be sad about. The episode "No Quarter" had him say rather bitterly, "And the punchline was the Blackout. When the world went back to being one giant schoolyard, and the Billy Underwoods are in charge and I am weak and afraid."
Has the Motor Mouth paramedic Denise, who keeps telling jokes and funny anecdotes as a way to cope with her son's death.
Bob Kelso is similar. He's a Jerk with a Heart of Gold who deliberately makes himself the one everyone hates so that they can put up with the fact that medicine is difficult. He's a Deadpan Snarker who pretends not to give a crap, but it's really hard for him to be the object of hate and make the painful money vs. life decisions.
The Sopranos: While talking to his therapist, Tony Soprano describes himself as a "Sad Clown": putting on a happy, joking face to his family and friends while keeping his pain locked away. His claims come across more as self-pitying than anything else, given his behavior throughout the series.
As Season One progressed, Dean went from pure comic relief to being a sad, lost little boy who really wanted his family back together and who wise-cracks only to mask that nasty pain.
Trickster/Gabriel. He ran away from Heaven to escape the fight, spent a very long time teaching people the error of their ways through wicked deadly tricks, only for it to be discovered that, behind all the black humor, he was really just scared to get involved. All in all, he's just a very heartbroken kid who invested too much of himself in his family to watch them destroy themselves. He spends much of his time onscreen trying to use the Winchesters to prove to himself that he was right to run.
The Twilight Zone: The episode "Five Characters in Search of an Exit" featured a memorable clown played by Murray Matheson with a healthy dollop of Mood Whiplash.
Clown: I'm a clown. It's neither here, there, nor anyplace... I could be a certified public accountant, a financier, a left-handed pitcher who throws only curves... What difference does it make?
Hank on Breaking Bad is introduced as a loudmouth, boisterous, almost oafish DEA agent. Starting around season two, he starts suffering PTSD and panic attacks from the stress of his job, but keeps up the same old persona in front of almost everyone.
Smokey Robinson and the Miracles have two songs which fit this trope: Tears of a Clown ("ain't too much sadder than the tears of a clown when no one's around") and Tracks of My Tears ("Take a good look at my face - you'll see my smile looks out of place. Look closer, it's easy to trace the tracks of my tears.")
Arnold Schoenberg's Pierrot Lunaire, a sad and lonely clown exiled on the Moon embarking on an eerie and symbolic journey. This character, from Belgian poet Albert Giraud, has been a fascination of early 20th century composers: Karol Rathaus composed a ballet about him.
Launcelot in The Merchant of Venice. His angst about wanting to leave his master vs. wanting to do the right thing gets Played for Laughs, as does his resentment of his father (who apparently cheated on his mother) and his sorrow at having to say goodbye to his only friend, Shylock's daughter Jessica. Alternate Character Interpretation has led to many a production implying that he's in love with Jessica, and he masks his sorrow that she chose Lorenzo over him by making some particularly cutting jokes about her parentage (as well as repeatedly trying to one-up Lorenzo in battles of wits).
Turns up pretty often with Cirque du Soleil, which has a habit of blurring the lines in traditional circus roles. The clown at the center of the "Snowstorm" act in Alegria, who makes and loses a friend that manifests him/herself in his hung-up hat and coat, comes to mind.
In the 1914 play He Who Gets Slapped by Leonid Andreyev, Baron Regnard plagiarizes the title character's work and steals his wife. To hide from the pain of this experience, "He" runs away to France and joins the circus as a clown. In his act, he plays a great philosopher who is slapped by the rest of the troupe.
Feste from Twelfth Night is very often played this way, although there's no real evidence for it in the text. It's particularly common for his final song ("For the rain, it raineth every day...") to take on a melancholy slant.
Zelos from Tales of Symphonia - seems the most cheerful and confident, but he's probably the most messed-up of the lot. And that's saying something.
Ace Attorney's Moe is definitely a good guy at heart, but his jokes are bad and everybody ends up hating him. The problem was that he was so stressed in court he veered wildly between trying to turn his testimony into a comedy act and behaving like a put-upon child. Outside of court he's at least tolerable and there are flashes of maturity there. The events of the case affect him, and he decides to take on the responsibility of being the ringmaster.
Luke Atmey in the third game describes himself at this. He may actually fit the definition, though not in the sense that he intends. It's all in the name.
Junpei from Persona 3 makes terrible jokes, has all the sex appeal of a snail, and sometimes displays extremely poor judgment when fighting Shadows. However, all of this is simply an off-shoot of his personal insecurity, as he is fully aware of his own limitations and believes that he will never make anything out of himself in life. Similar to Yukari, he also comes from a one-parent household, and lives in the dormitories just so he can get away from his alcoholic father.
Anders from the Awakening DLC expansion puts up a pretty glib front, but doesn't exactly appreciate having been taken from his parents and locked in a Mage Tower (as evidenced by at least seven escape attempts). If you suggest mages should just accept the system, he reacts very angrily, and if you befriend him he half-seriously admits part of him wants to "rain fireballs on every templar in creation". When he returns in the sequel, he makes good on this.
Seemingly made canon in the third game. Shepard is clearly not fine and under a ridiculous amount of stress. Even Anderson, who's back on Earth moving from fox-hole to fox-hole whilst fighting the Reaper invasion, seems more concerned about getting people to look after Shepard.
Pretty much any teammate who makes jokes can be considered this, due to the fact that they all have issues of their own, and the entire galaxy's going to hell. KasumiGoto is possibly the only joker who is an exception to this, as even though she does have problems in her past, she genuinely seems to have moved past them (unless she's just really good at covering them up).
It's implied that deep down, Kasumi isn't over the death of her lover, Keiji. Depending on how her loyalty mission ends and if Keiji's greybox was saved, Shepard will repeatedly express concern in 2 and 3 that Kasumi is spending too much time watching his memories and not enough making her own.
A clown in Fantasy Quest turns out to be sad indeed, mostly from his lack of a nose.
Sean O'Cann of Survival of the Fittest, to which the above quote applies almost perfectly. Prior to the point in the game (Day 3) that he found out his best friend, boy friend, and cousin died (three different people, before anyone says anything) he still cracked a joke every now and then. Afterward though, Sean begins making all sorts of remarks, not all of which are in the best taste, and sometimes are just plain offensive.
Evan in Everyman HYBRID is the most affected by the Sanity Slippage caused by the group's dealings with the Slender Man and other assorted problems; he also tries to lighten pretty much every situation, cracking jokes while exploring creepy abandoned buildings or even while dealing with a monster literally hiding in a closet at a friend's house. The latter is likely the best example of his humor backfiring for those around him, as he pulled this after said monster violently assaulted his friend's brother.
Related to the example below, a lot of the comedy (the character-based, at least) in The Nostalgia Critic is based on how unhappy he is with his life.
Dragon Ball Abridged: Cracking awkward, horribly inappropriate jokes seems to be how Krillen deals with all the crap he's put through.
Phase of the Whateley Universe. Since he narrates his own novels, the reader gets to see just how emotionally damaged he really is, even though he refuses to admit it to anyone (except the school shrink).
Teen superhero Clockblocker of Worm is this, as revealed to his therapist.
Aang from Avatar: The Last Airbender. His cheerful exterior and fun-loving personality is a way to cope and ignore the guilt he feels for running away from his home and people and getting frozen, allowing the Fire Nation to wage war for a century, wiping out his people. He finally faces his guilt later on when he's mentored by Guru Pathik.
Iroh arguably counts as well. Underneath his goofy facade, he's just a father grieving the loss of his son.
Beast Boy from the Teen Titansanimated series is an insecure and vulnerable kid who constantly uses humour as a defense mechanism.
X-Men: Evolution has Kurt Wagner/Nightcrawler, who may as well be the poster-boy for this trope. He's possibly the most sensitive, emotionally vulnerable character on the show (next to Rogue, his adopted sister), and spends a good deal of his time angsting over his appearance (in early episodes) and his mother, Mystique (in later episodes). Nevertheless he is the resident "goof-ball", trying his best to appear the carefree joker when deep down, he's anything but carefree..
Plastic Man: C'mon, Bats. No one's ever wanted me to be a part of their team. Even the League threw me out. Give me a chance to be a part of something - to prove to myself that I'm not a three-time loser.
Notably not the Flash on Justice League, as illustrated nicely in the episode Flash and Substance. Some people just don't seem to get it.
Orion: I understand now. You play the clown to hide a warrior's pain. Flash: Dude... The bad guys went down, and nobody got hurt. You know what I call that? A really good day.
It is possible that the high-energy lifestyle and constant peppiness could be a result of him being aware of how dangerous his powers actually are to himself and the people around him, but it's less about masking angst and more about thoroughly enjoying every moment he gets.
Some fans of My Little Pony: Friendship Is Magic have pegged Pinkie Pie as one of these. In "Party of One", the constantly-cheerful party pony is quick to fall into a funk when she thinks her friends don't want to come to her parties or be friends with her anymore.
In a more literal example, we have Krusty the Klown from The Simpsons
Homer: Let's tell Krusty! That guy's hilarious! Marge: I keep telling you, off-camera he's a desperately unhappy man.
Ultimate Spider-Man: Spider-Man's wacky jokes and quirky personality are what he uses to shield him from a relentlessly unkind world.
"This right here is the difference between paycheck and nightly bed check."
Russell Brand. He was diagnosed with bipolar disorder, and had bulimia as a child. He also used to have terrible cutting incidents and numerous addictions. As he himself puts it, "My biggest problem is that I have lived an autobiography rather than a life."
John Lennon used his reputation as "the witty Beatle" as a mask to cover up his massive insecurity, to which he even confessed in "I'm A Loser" and "Nowhere Man" - they came to him after five frustrating hours of writer's block.
Chilean lawyer and reality show star Juan Cristobal Foxley, nicknamed "Dandy Chileno" ("Chilean Dandy"). Article in Spanish is here.
Richard Lewis. As if it wasn't obvious before he publicly admitted his problems with depression and began making an effort to help others with the same problem.
A similar thing was done by New Zealand comedian Mike King - who previously experienced substance abuse and depression - in the form of the Nutters' Club radio show, where listeners call in to explain their mental health issues.
Gene Wilder. Incredibly so. One can even sense a profound sadness in his hammiest performances.
The Great Zucchini. Wildly successful children's entertainer for people like Sasha and Malia Obama on the one hand; deeply indebted gambling addict desperately trying to hold his life together on the other hand. Thankfully, he got better.
P. G. Wodehouse parodied this in the foreword for The Clicking of Cuthbert, an anthology of golf-related stories, where he wrote that he didn't fit this trope before, but now he does, because he started playing golf.
Most people on That Guy with the Glasses are like this. They've gone through depression, abusive relationships and illnesses, but use comedy to tear those taboo subjects down.
Soviet 1920s-1930s satire writer Mikhail Zoshchenko suffered from severe depression. His semi-autobiographical book Before Sunrise was initially banned by the Soviet censors, as its themes of depression ran against the triumphalism and optimism of Socialist Realism.
Dave Chappelle, who disappeared from the public eye mostly after his show ended over becoming disinterested in his own material.
When he was in Black Sabbath, Ozzy Osbourne was considered the "clown" of the group, constantly playing pranks on the other members and performing outrageous stunts and parties to get attention. Once he went solo he became famous for his debauched personal life and shocking onstage antics. His public persona is mostly an act, and in his personal life he has often struggled with depression, anxiety, and alcoholism.
Stephen Chow is best known for his comedic roles, but away from the camera he's known for being surprisingly humorless and lethargic. A troubled and impoverished childhood might be the cause.
Darrell Hammond of Saturday Night Live is famed for his wide variety of impressions and being on SNL for fourteen years (longer than anyone else). His backstory reveals a long history of extreme parental abuse; his mother has been mentioned as slamming his fingers in car doors as a child and cutting his tongue. In fact, he started doing impressions to please his mother to avoid the abuse. Alcoholism, cocaine addiction, and mental illness (he suffers from bipolar disorder, had a long history of self-harm and has sought treatment for schizophrenia) are also part of his backstory. Remarkably, he has still spoken very openly about it and is far more well-adjusted than anyone in his position should be expected to be.
Kevin Macdonald and Dave Foley of The Kids in the Hall are both this to varying extents. Mac Donald was struggling with depression and a divorce while making the film Brain Candy (ironically about a drug that makes people happy). Foley has been like this since his marriage collapsed, his career declined and he started owing hundreds of thousands of dollars in child support. If he ever returns to his native Canada, he could very well be arrested.
John Cheese of Cracked has long been like this due to his struggles with alcoholism, poverty, parental abuse and depression. Despite this, his writing often reflects on such things in an extremely funny light and he has repeatedly claimed that he doesn't blame others for his problems and uses his writing to let people in similar situations know that they are not alone and things can get better.
Marc Maron due to his struggles with Mental illness.
David Lange, the Prime Minister of New Zealand from 1984-1989, attributed his acerbic wit to the need to defend himself from being bullied about his weight.
Matthew Perry, much like his character Chandler Bing who is listed in the TV section. While he played pranks and cracked up the audience on the show, at home he battled a horrible drug addiction and alcoholism which became in his words "a matter of life and death". Even his notoriously close cast-members were unaware of how unhappy he was. He eventually admitted his problems, checked himself into rehab and since then has been a public figure in raising awareness for drug addictions.
Drew Carey has struggled with bipolar disorder for years and has attempted suicide twice due to the severity.
Rodney Dangerfield. Good lord, the man just made you want to hug him and tell him "It will all be okay." His memoir "It Ain't Easy Bein' Me" is unbearably sad, detailing his emotionally abusive parents, his initial failure to break into show business, his lifelong struggle with depression, and his genuine self-hatred which he turned into self-deprecating one-liners.