In Madlax, the eponymous heroine is a gun-for-hire in a civil war-torn country, yet this only makes her more appreciative of life and its small everyday joys; e.g. she visits her client and target (same person) on the night before his assassination to comfort him. In the end, it is she (or the part of Margaret corresponding to her) who defeats the Nietzsche Wannabe of a Big Bad.
Elmer C. Albatross of Baccano has pretty much this exact outlook. The guy had a horrible childhood and as a result adopted this sort of unsettling Stepford Smiler personality and obsession with happiness. Basically, he feels the world sucks so much that it's important to be happy.
Kaji also has very strong traits of it. He knows more about what's going on than almost anyone else, yet he's the only character who appears genuinely happy. During one very close battle against an angel that appears to be the final moments before the end of the world, he is watering the melon patch he is growing, with the battle being visible in the distance. If the world does not end on that day, then the melons need to be watered. If it does, then it won't matter what he is doing in the final moments anyway. Either way, he can't do anything to change what's going to happen in the next ten minutes.
The character Panaru in episode three of Boogiepop Phantom is respected for having this philosophy and teaching it to others.
Makina from Shikabane Hime. The final scene in the anime is of her repeatedly punching her archenemy Hokuto (a fellow fighting-zombie) in the face after all is lost: it's the first time she ascribes meaning to her existence.
Makina: You're not an undead! You're alive! And so am I!
Main point of Kino's Journey: "The world is not beautiful, therefore it is".
"Even though this world is without witches, that doesn't mean there are no curses. The distortions of the world change form and target people from the darkness. This may be a world without salvation and where nothing but sadness and hatred repeat, but it's still a places she once tried to protect. That something I remember, something I will never forget. That's why... I will keep fighting."
Lelouch from Code Geass ultimately believes in people's desire for the future.
Optimus Prime becomes this in The Transformers IDW. This might also be a case of Reality Ensues as to the effects of being at war for millions of years would have on one's psyche.
He's slightly different from the example, but Rorschach has a similar philosophy. Instead of abandoning rules and discipline due to a nihilistic outlook he decides his rules and principles are all the more important in a world that has no more meaning than the one we impose on it.
Rorschach is arguably more of an Übermensch, considering his largely misanthropic view of life. It's shown that he does care about what few friends he has in the world, though. YMMV
Dr. Manhattan also develops into this by the end.
Rorschach's therapist, Malcolm Long, who becomes influenced by his philosophy, also comes to a similar conclusion. When his wife tells him not to intervene in a street fight, he says: "I have to. In a world like this... I mean, it's all we can do, try to help each other. It's all that means anything..."
Batman, Depending on the Writer. The victim of a random and meaningless crime, young Bruce Wayne could have decided that life was pointless, and succumbed to depression and nihilism. But instead he chose to create his own purpose, re-inventing himself as a force for order and justice, which stands in direct opposition to The Joker's strain of chaos and destruction.
There's actually a surprisingly deep quote in Batman & Robin which captures the existential nature of Batman's character.
Alfred: "Death and chance stole your parents. But rather than become a victim, you have done everything in your power to control the fates. For what is Batman if not an effort to master the chaos that sweeps our world, an attempt to control death itself."
Despite hanging out with various Norse gods and calling Thor "oathbrother", Beta Ray Bill does not believe in God in the theological sense. This doesn't stop him from being a true hero.
"If there is nothing but what we make in this world, brothers... let us make good."
Transformers More Than Meets The Eye: Minor character Ore was killed in the first issue, the annual (coming out between issue 8 and 9) has him temporarily revived, and his dialogue reveals him to be one. At one point Swerve asks if he's a believer of in any science or religious answer to life.
"I'm not anything. I just think... pfft. What do I think? I think that life is violent and cruel -and precious. Yeah... I think you don't have to believe in a higher power to be overawed by the world around you."
Harry himself, in Harry Potter And The Methods Of Rationality. He knows that there is no God or meaning to the universe, so he decided to become God, and... optimize things. This is also the fundamental difference between him and this 'verse's Voldemort. Voldemort saw an uncaring world and said "Why not be evil?" Harry saw an uncaring world and said "Why not be good?"
Collateral shows how one of these might be created. Max starts the film as an idealist and a dreamer, contrasted with Straw Nihilist Vincent. After a series of lectures from Vincent, Max eventually throws his philosophy back in his face—yes, Max's life is meaningless, and yes, if he continues living it the way he currently is, he'll never achieve his dreams, so why shouldn't he risk his own life to try to stop Vincent?
A number of protagonists in Woody Allen films are people who don't believe there is much more to life than misery and unhappiness, but desperately search for meaning in their existence, and/or simply find a way to enjoy themselves while they can. They may also seek to "escape" their reality, either through overactive imaginations (some of Alvy's childhood memories in Annie Hall, for instance) or more mysterious, generally unexplained Plot Devices (Midnight In Paris).
In Annie Hall, a number of Alvy's opinions on life and relationships reek of pessimism and nihilism, best exemplified by the film's opening lines,
Alvy: There's an old joke, um... Two elderly women are at a Catskill Mountain resort, and one of 'em says, "Boy, the food at this place is really terrible." The other one says, "Yeah, I know; and such small portions." Well, that's essentially how I feel about life—full of loneliness and misery and suffering and unhappiness, and it's all over much too quickly.
This, however, is turned around when Alvy shares another joke at the very end of the filmnote This guy goes to a psychiatrist and says, "Doc, my brother's crazy; he thinks he's a chicken." And, the doctor says, "Well, why don't you turn him in?" The guy says, "I would, but I need the eggs." and rationalizes that he has a reason to endure all of the absurdity and suffering "because he needs the eggs". The theme is also exemplified through Dr. Flicker's viewpoint in his talk with young Alvy and his mother, when young Alvy had concluded that nothing is important because the the universe is expanding and everything in existence will fall apart.
Dr. Flicker: It won't be expanding for billions of years yet, Alvy. And we've gotta try to enjoy ourselves while we're here!
In Midnight In Paris, Gil finds his reality to be unsatisfying and his work as a Hollywood screenwriter to be worthless. He wishes to escape it all, and he does. However, he later concludes that there is no escape as life and present-day realities are always unsatisfying for everybody, and one has to live with it. The story concludes with Gil, having decided to leave his "Golden Era", finally finding meaning and joy in his own contemporary era.
In Stardust Memories, Sandy Bates is a director of many comedy films who started to tell more serious stories because he started finding life to be too miserable and full of suffering and nothing seemed funny to him anymore. His latest movie is about delivering a message along the lines of, "No matter who you are or what you did, your life is headed for a garbage dump." He desperately searches for meaning in life, discovering that he'd find it in love and enjoyment in such, despite maintaining that everyone is just headed for the metaphorical "garbage dump".
Phil in Groundhog Day, eventually. Nothing seems to keep the day from repeating, so might as well help out.
The title character of Jacques the Fatalist is (obviously) The Fatalist, and since he believes everything that happens is preordained, he appreciates the good things and reacts with stoicism toward the bad ones.
Attila József: "Why should I be honorable? I'll be laid out regardless! Why shouldn't I be honorable? I'll be laid out regardless." ("Miért legyek én tisztességes? Kiterítenek úgyis! Miért ne legyek tisztességes? Kiterítenek úgyis.")
Arguably, Ralph, Piggy and Simon in Lord of the Flies. The primary theme of the novel is the regression of humans when removed from the rules and order of society. Ralph and Piggy ruminate on the fact that structure and meaning of the "real world" doesn't apply to their island, but they still commit themselves to maintaining order and peace. All the others, without anything to control them, quickly fall into anarchy and violence.
The books of His Dark Materials end up coming around to this theme, more or less, with a bit of a Fantastic Aesop courtesy of Dust. But it's former nun Mary Malone who first reaches the conclusion: that though once she felt that no God meant no purpose to the world, the need to keep Dust alive, that is, wisdom, curiosity, education, and kindness, gives the worlds purpose. In other words, "There is now!"
Christine, the narrator of the novel Dead Romance thinks like this when she's dismissing one of her friend's uber-depressing, Wangsty poetry:
What I'm getting at is that in a pointless, empty universe a good time is as meaningless as a bad time, so you might as well slap on a smile and get on with your life.
Professor Pangloss in Voltaire's Candide, who goes through Hell and back and comments on the weather.
The narrator of Ted Chiang's short story "Exhalation" is watching his universe wind down to equilibrium. Rather than despair, he implores future explorers to "contemplate the marvel that is existence and rejoice that you are able to do so. I feel I have the right to tell you this because, as I am inscribing these words, I am doing the same."
Mal fought for freedom and honor in The War Of Coreward Aggression. He lost, and has come to terms with that. But at the same time, he refuses to be a slave or a thug - even when the entire 'Verse insists that he has to obey a higher authority or act against his principles to survive, he remains Captain Malcolm Reynolds. And he aims to misbehave.
Simon also has something of a tendency toward this. He specifically states that acting morally means even more out in the black without an authority to impose it.
River recognizes that all meaning is "imbued" and thus there really is no "meaning" to begin with, River has a surprisingly positive outlook on life, and sees things in a very innocent way (i.e. the loaded gun everyone was freaking out about took the form of a harmless stick in her mind). Objects In Space is actually an exploration of these two character types, juxtaposing River against Jubal Early, who's definitely a Straw Nihilist. Faced with the same realization as River, Jubal's response was to become a complete psychopath who tortured his puppy.
Angel from Buffy the Vampire Slayer once had the revelation that life has no purpose or meaning, thus making even the tiniest act of kindness an end in itself. Angel's Koan:
If nothing that we do matters, then all that matters is what we do.
Wesley outright states the existential nature of the character Angel (as well as the series itself) when he says:
"There is a design Angel, hidden in the chaos as it might be. But it's there. And you have your place in it."
Contrary to Angel, his son Connor also encounters an existential crisis, but he acts as a mirror image to his father by going the opposite route and becoming a Straw Nihilist.
Inherent in Star Trek. The Federation - or at the very least, Earth - is a whole society of Anti-Nihilists. Since there is no scarcity, the acquisition of wealth is pointless*
(But whatever you do, don't tell the Ferengi that. They'll burn you for heresy!)
. Since they've Outgrown Such Silly Superstitions, religion is irrelevant. And since the universe is big enough for everybody, there's no reason to conquer or force people to serve you. Life is focused on individual accomplishment and/or happiness. People go out and find their own purpose - whether that might be exploring the galaxy, making great scientific discoveries, or just starting a farming colony.
Van Der Graaf Generator tackles the subject with "Lemmings"; the first half dealing with the meaningless of life, and the second concluding that ending it all won't fix anything.
The Bleak Cabal in Planescape is canonically an example of this. Life is meaningless and cruel so hey, no need to add more meaningless cruelty to it by your own actions. The Bleak Cabal runs Sigil's soupkitchens and asylum, and are at a whole a rather decent bunch even if most of them are insane to one degree or another. Sure, their actions won't make any difference in the long run (but in their view, nothing does anyway), but it helps today.
They may be the only example on the page that exist in the afterlife, surrounded by evidence of gods, and still hold to this philosophy. Of course, beholding a being of divine power and acknowledging it as an empirically superior being are two different things.
The more idealistic characters in Exalted. Sure enough, the world is gangbanged from all directions by undead, Wyld mutants, demons, and other awful things. The folks in charge of defending it are too busy politicking. Heaven is a sham and a scam, and the patron god of heroism is a crack addict. But —and this is a massive but— you're a hero, possessed of a power to drastically change the world. And by "hero", it's hero in an archaic sense: you get to decide what is right or wrong, answerable only to your own conscience (or the lack thereof).
Nurgle, the Chaos God of disease and despair in Warhammer / Warhammer 40000 is, suprisingly enough, like this. Yes, all things will eventually die and rot, and you will most likely never achieve your dreams, but does that matter? Instead of angsting over it or spending you life trying to reach an impossible goal, you should just be happy with what you have. Nurgle loves you no matter what you do. Of course he is the god of disease, so while he genuinely does care for you, his idea of caring involves "gifting" people with terrible diseases...
This is the life philosophy of the Geth in Mass Effect. The usual philosophical conundrums of organics are easily answered by the Geth: Why do we exist? What happens after death? For the Geth, they were created to be menial labor for the Quarians, and their memories are archived after "death". Since they are no longer performing their original task and have been "disowned" by their "gods", they have created their own purpose: total unity of ther Mind Hive in a single giant "Dyson Sphere" platform (though a more accurate analogy would be a Jupiter Brain). The True Geth (which Legion is from) want to do so on their own terms along technology paths they find themselves, while the Heretic sect took an easier, more controlled path under the command of the Reapers.
The Assassin's Creed; "Nothing is true, everything is permitted." There is no God. There is no Devil. There are only Flawed Humans, the children of flawed Precursors. So if we wish to live in peace, prosperity and freedom, we must build a civilization that permits those things.
Altaďr Ibn-La'Ahad: ...laws arise not from divinity, but reason. I understand now that our creed does not commend us to be free - it commends us to be wise.
Ezio Auditore da Firenze: ...merely an observation of the nature of reality: To say that nothing is true is to realize that the foundations of society are fragile, and that we must be the shepherds of our own civilization. To say that everything is permitted is to understand that we are the architects of our actions, and that we must live with their consequences, whether glorious or tragic.
Solid Snake of the Metal Gear series adopts this outlook. He doubts whether or not his actions will make a difference in the long run and knows how brutal and senseless life can be, but still devotes himself to the cause of creating a more just, peaceful world by stopping the proliferation of superweapons, especially the eponymous Humongous Mecha.
In Fate/stay night, Shirou lives his life by this sort of doctrine, even though he doesn't realize it. The "Unlimited Blade Works" route causes him to actively seek it, as he realizes his life is empty and the only thing he finds meaning is is selflessly sacrificing himself for others.
There is a xkcdstrip in which the white beret guy used this trope when confronted with a more typical nihilist.
ThisA Softer World strip sums it up pretty well. "Nothing matters at all. Might as well be nice to people."
When the Shadowchild asks her what the importance of being good is, Digger replies (paraphrased), "Because life's hard enough as it is."
Soren Kierkegaard's "Knight of Faith", one of the major influences to the Existentialist trope. You fully accept that following such stuff about God and ethics are ultimately an absurd goal in this life, but in contrast to the aesthetic "nihilist" and the otherworldly "knight of infinite resignation", you still prefer to make the most virtuous out of it.
Indeed, many if not most atheists embrace this as their world view in place of any belief in a higher power or afterlife, calling out critics on why they think only Big Brother can prevent them from becoming Complete Monsters (see also: What You Are in the Dark).
Friedrich Nietzsche's version of the Anti-Nihilist in particular is less of a "utilitarian" who works For Happiness and more of a "Let's live by our own rules while being awesome, manly/virtuous and magnificent at it". It should also be noted that he pointed out that a way to make life suck a little less could be through charitable acts (note that he didn't say that it would make you happy, just feel less angsty/guilty/whatever)
His idea was more the strong protect the weak. The enlightened would rule benevolently.
There might be older instances of this trope that couldn't be properly called Nihilists, since they predate the movement, but they should go through a Discussion first, so as not to dissolve the trope into meaninglessness.
Albert Camus, despite The Stranger. In his essay The Myth of Sisyphus he argues that the absurdity of life is in fact an invitation for human beings to experience revolt, freedom and passion and not in fact a valid reason to commit suicide (which he argues is even more absurd than life itself), concluding with the thought that "The struggle itself [...] is enough to fill a man's heart. One must imagine Sisyphus happy."