The Wonderful Wizard of Oz was, in his debut book, most known for being a humbug, and more a plot device than a character. His return to the series in the book, Dorothy and the Wizard of Oz made him into an Action Hero, dual-wielding swords against all manner of monsters on his way back to Oz.
Despite many of the Discworld's more iconic (and recurring) characters having a mostly constant level of badass (Vimes, Granny Weatherwax, Rincewind, Death, the Librarian) certain Discworld characters gain quite a few levels by the end of their books. Among the more notable are Brutha (Bishops move diagonally) and the great god Om from Small Gods.
Vimes wasn't badass in Guards! Guards!, but in Men at Arms he was seriously Badass. It could be argued that he took another level after Men At Arms, as well: in the other Watch books he is probably the most badass thing on the Disc.
Susan Sto Helit, Death's Granddaughter has taken more than one level in Badass over the course of the books she's involved in. Most spectacularly was against Mr. Teatime in Hogfather. Tip: Don't piss off Susan when she's near a poker.
The wizards of Unseen University tend to both play this trope straight and then almost immediately subvert it. Nearly all of them, everyone from Archancellor Ridcully, down through Ponder Stibbons and his High Magicfaculty to the Bursar take a level in Badass. However they will nearly always revert back to their Squishy Wizard selves in the end.
Magrat takes a sequential level in Badass every book. Her finest example is in Lords and Ladies. If you don't know as to what I'm referring then please either see that page or Discworld's Awesome Moments page.
Rincewind could be described as a Stealth Badass. On the surface he is cowardly, incompetent and all-around useless ... but no matter what terrible situation he is unwillingly thrust into, he always comes out unscathed.
Joshua and Dora in Dora Wilk Series. He goes from The Unfavorite angel of his grandpa to healer, wielder of archangelic swords and one of few angels who can Manifest. She goes from half-blood witch living outside of her world to immensely powerful wielder of all magics and one of the most important figures on Thorn's political scene.
Nathaniel Starbuck begins the novel Rebel a jilted, humiliated, penniless son of an emancipation preacher who has been caught by a mob in Richmond and has to be rescued by his friends. The book is about his taking a level in badass after joining the Confederate Army. The definitive moment comes when he has to arrange rail transport for his regiment and the controller isn't willing to lend him the cars necessary:
Nate Starbuck: But I am. (*shoots wall beside him)
All five main characters of Animorphs start the series as ordinary kids and over the course of fifty-four books gradually become hardened guerrilla fighters. Sometimes this isn't always for the best — Rachel becomes a scaryBlood Knight.
Of the five none take more levels in badass than Jake. By the final arc he's become the most effective leader in the war, outwitting both his rival general Visser One and the Yeerk traitor infesting his brother. Near the end Visser One's host explicitly states that Jake is the only person the Visser is afraid of. Not bad for a 16 year old.
Sixth Ranger Traitor David also gets a special shout-out. He has only three books to grow in, but during them he goes from a kid with a BB gun and a pet cobra to a foe so effective he comes closer to defeating the Animorphs than the entirety of an alien empire. The Threat ends with him defeating Jake in a duel between his lion and Jake's tiger and he's the only enemy in the entirety of the series who makes Blood Knight Rachel feel fear.
Before Astrid Ellison was put on a bus in PLAGUE (4th GONE book), she was as dependent on her boyfriend Sam as a pre-feminist 1950s sitcom wife...She comes back 4 months later after living in the woods, smoking pot and apparently kicking any ass that comes her way, as she's apparently pretty violent now.
Diana Ladris did the inverse; whilst Astrid started out as a defenseless, pathetic damsel and evolved into a dangerous, quick-witted Action Girl, Diana started off as an evil badass who was smart and strong enough to get herself out of any trouble... Then she gets pregnant, and apparently that makes her a helpless little infant now.
Neville Longbottom, previously a bit of a Butt Monkey, gets some character development in Book 4, then actually starts showing some competence in Book 5. This pays off in Book 7; the Power Trio misses out on what he's up to for much of the year, but when they see him again, he's clearly leveled up in Harry's absence and has become a leader in his own right. He's a key player in the final battle, and actually ends up destroying the last Horcrux: he pulls the Sword of Gryffindor out of the Sorting Hat and uses it to lop the head off of Nagini, leaving the way free for Harry to kick Voldemort's scaly ass one final time. Did we mention Neville was on fire during all of this?
The fandom has turned this up to about 90, with things such as "What happens if you break the Unbreakable Vow? Neville."
Neville is now the series' designated Memetic Badass. That's a big development from how he started. His offscreen appearance in the widely reviled epilogue may be the only aspect of this segment more likely to invoke a readerly "Yeah!" than a readerly yawn.
It's even better in the Jim Dale Audiobook of Order of the Phoenix, because he starts sounding tougher once he gets a BROKEN NOSE. It's as if he's bleeding not blood, but wimpiness, leaving nothing but the ultra badass left.
Harry himself takes one of these, going from bullied and timid kid to a pretty reasonable all round badass by book 4 at the latest. You could argue he takes a further one of these in the final book, going to his death calmly, then just as calmly offering the man who killed his parents and a lot of his friends a chance at redemption, admittedly almost certain that he wouldn't take it.
Ginny gets several, first going from the shy girl who can't speak in front of Harry and gets possessed by Lord Voldemort and can't tell anyone no matter how hard she tries to the more confident teenager seen in Goblet and the early stages of Order. Then she levels up when she joins the D.A., starts training and then fights in the Ministry. Then, in Deathly Hallows, she's helping to run the D.A. and insists on fighting in the Battle of Hogwarts, against the express wishes of her mother (backed up by her father, Lupin and Harry). And then she fights Bellatrix.
Hermione and Ron also take levels in badass, but less markedly than the above examples.
Same with a good many of the secondary characters. In the first book, it's made clear that most of the new students at Hogwarts could do hardly any magic at all. By the last book, the same students are participating in a battle with full-grown, extremely dangerous criminals, and holding their own.
Happens to Admiral Daala from the Star Wars Expanded Universe. In her original appearances, during the 90's, she was an overblown officer suffering from terrible levels of Informed Ability, repeatedly stated as being a tactical genius, but her strategies were highly incompetent. On her return in the Legacy of the Force series, she not only lives up to the reputation she was given in-universe, but kicks all kinds of ass using a fleet consisting mainly of obsolete starships. On top of everything else, she has an eyepatch now, and a full name. Natasi Daala
Pellaeon himself is also an example of this trope. In The Thrawn Trilogy he is 'merely' capable, a reliable and hard working Star Destroyer captain and little more. Yet through learning from Thrawn and Daala and through sheer weight of experience he becomes quite a wily and formidable commander until his Crowning Moment of Awesome decades later in the New Jedi Order when he defeats an enemy fleet while floating in a bacta tank. The Star Wars Expanded Universe is full of brilliant admirals and commanders but Pellaeon is probably the only one who made it to the top through sheer hard work rather than being a born genius.
In the Doctor Who New Adventures novel First Frontier, the Master discovers the hard way this has happened with the Doctor and Ace since he last fought them. He wipes the floor with some clones of them created by his allies and gets a fancy new pure Time Lord body with a new regeneration cycle just in time for a rematch with his ex-BFF. The problem? Well, the Doctor's become a ruthless Chessmaster, Ace has spent several years blowing up Daleks on the frontline of future-Earth's war with them, and new-ish companion Bernice has been conditioned by the Doctor to respond to the Master's usual hypnosis attempt with a big WTF? To add injury to insult, the second Ace realizes it's the Master, she shoots him in the head, forcing him to use up the first of those new regenerations mere hours after acquiring them.
Gandalf's evolution from Gandalf the Grey to Gandalf the White. He's granted the ability to use more of his natural power while acting as Gandalf the White. In a popular parody Russian redub of the series, Gandalf in fact says, "I fell into the white and gained a levelup" upon his return.
All the hobbits take a level in badass throughout their adventures, going from idle country folk to heroes whose various exploits help save Middle Earth. "The Scouring of the Shire" shows their evolution, as they return to their old home and must give the other hobbits a quick lesson on the badass that they've learned. Hobbits as a species have a predisposition toward this trope, summoning their "hidden depths" and rising to great challenges.
The Hobbit contains one scene where Bilbo vanquishes a giant spider. The description of his newfound confidence afterward practically qualifies as gaining a Character Level.
Thorin and the dwarves, too. They spend the book getting caught by trolls, caught by goblins, caught by wolves and goblins, caught by giant spiders, and caught by elves. But when the Battle of Five Armies is going down, with thousands of combatants on both sides, Thorin and his twelve companions (one of them comically obese) make a significant difference to the battle and almost cut their way through to the freakin' Goblin King himself. True, they had the pick of the armour and weapons in Smaug's legendary hoard, but even so...
In the Latin textbook series Ecce Romani, Sextus spends a whole two years worth of lessons being annoying, whiny, and cowardly. Then, being his usual wimpy self, he goes back to the changing room at the public baths because the water's too hot, sees a thief stealing his clothes and chases the guy halfway across the bath complex on slippery tiles, then pushes him into a frigidarium (that's the cold pool, for you non-Latin-students). He even joins the army in the Epilogue.
On the subject of Latin textbooks... the Cambridge series! In the first book, Quintus has such exciting adventures like going to debates and accidentally breaking statues in the public baths... then his dad gets killed in a volcano and all of a sudden he's a world traveler, fighting rowdy Egyptians, killing crocodiles, hangin' with kings, prosecuting corrupt officials, and just generally being ridiculously kick-ass.
The Laughter of Dead Kings, the most recent Elizabeth Peters Vicky Bliss book, reveals that Schmidt has had a hidden level of badass all along, and actually is the Greatest Swordsman in Europe.
In Prince Caspian, when the Pevensies return to Narnia, they regain all the fighting skills they had learned in the fifteen years they were in Narnia the last time. They return as children, except the relatively inexperienced children you had read about in The Lion, the Witch, and the Wardrobe are now a bunch of young badasses. The difference is even more noticeable in The Film of the Book.
In Malazan Book of the Fallen Ganoes Paran starts out as the Ensign Newbie and is treated as a tool by pretty much everyone in the setting. Eventually, the Azath Houses take an interest in him, and suddenly he's the one setting the rules the gods have to follow.
The Wheel of Time has this happen to most of the initial cast. They start out as random villagers, and slowly develop over the series.
Rand is consistently Leveling Up in Badass. The climaxes of the first five books, and most thereafter, are an expression of this leveling-up. The other primary channeling characters experience similar (though lesser) bursts of growth.
Magical reasons are sometimes given for the level-gaining. Mat Cauthon, for example has implanted fighting and tactical planning memories turning him overnight into a canny warrior and general. Rand al'Thor also has memories from one of his previous incarnations to help him along.
The only unexplained levelling up takes place with regards to Gawyn Trakand who goes from a decent swordsman to somehow being able to take down experienced Warders twice his age to later wiping out attacking bands of Aiel warriors, trained since birth to be badass, by himself.
Susan from The Dresden Files between Grave Peril and Death Masks. At the end of the former she turns down Dresden's proposal of marriage to leave Chicago to find a way to deal with her half-vampirism, which she had gained after being taken hostage to try and manipulate Dresden. When she returns, a combination of the increased physical capabilities granted by her state, and training and mystic tattoos given to her by an organization which is a combination of a support group for people like her and covert organization fighting against vampires leave her a competent combatant (who at one point is able to match a surprised sorcerer being powered by a fallen angel) who is a useful asset to Dresden during his case in that book. At the end, she returns to South America (where she had spent the interim between books) to take in the fight against the dominating vampire faction based there.
Do inanimate objects count for this trope? If so, Harry's shield bracelet between Proven Guilty and White Night took a couple of levels, going from a burnt-out talisman that only blocked kinetic energy and spat out sparks every time he used it to a shiny new trinket interlaced with several precious metals that blocks "heat, cold, electricity — even sound and light". Even (Elaine), whose specialty was in subtle and varied magics, was impressed by the versatility of it.
As does his rings, which can unleash a single powerful kinetic attack on a foe before needing to be recharged. He starts out with a single ring, then a ring with three bands, each band as powerful as the original ring, then starts wearing a three-band ring on each finger. Giving him a total of 30 times the kinetic goodness of his original single ring.
During the first few books Harry would burn out after a couple of spells. Now he can blast spells like crazy in a fight, due to his toning of his metaphysical muscles. And the firepower of his gun has gone steadily up too. He is becoming adept at Xanatos Speed Chess.
He has gained better control over his spells as well. Originally defined as a 'magical bruiser' with power but little fine control he gains finer control and ability at subtler magic as the books progress, particularly after he started teaching Molly. He wouldn't even bother trying a veil before, but he had to brush up on them fast to avoid looking like an idiot in front of the Master of Illusion.
He's also gotten more and more help from supernatural entities over time; Lash gave him Hellfire and some useful information, then, after her Heroic Sacrifice, Uriel grants him access to soulfire. And then Harry makes a well of dark power into his sanctum. Seriously, Harry's taken more levels in badass than a shonen hero.
Changes: Sir Harry Dresden, Winter Knight.
The Alphas. They go from a bunch of high-school senior/college age nerds who wear too much leather and Old Spice and just happen to be werewolves to a pack of fit, healthy young men and women who transform into a virtual army of monster-shredding fangs and claws; enough to strip a professional ghoul assassin to the bone. Mentioned by Harry in Summer Knight.
Murphy but her increase in badassness has less to do with getting more badass and more knowing how to be badass to the latest Monster of the Week. Storm Front, does nothing. Fool Moon, wounds a loup-garou with a pistol. Summer Knight, defeats an ogre and a plant monster with a chainsaw.
Small Favor, drives off a Denarian by pulling Fidelacchius two inches out of its sheath.
Changes, cuts a bloody swath through the Red Court, looking like an avenging angel complete with glowing gold halo, before One Hit Killing a Physical God.
Molly, finally takes a level in badass in Changes, despite still being one of the lesser badass's on Harry's side. While good at the subtle magic her inability to use the more powerful offensive magics left her a non action girl for most of the series; relegated to veiling herself and others who need to hide. By Changes she finally takes a level by first helping two others to fight and drive off a house sized demonic dog-thing that had already taken Harry out by popping in and out of veils to bait and distract the creature and later in the final climatic battle using her magics to create such bright lights and sounds that she could daze and disorient attacking vampires to prevent them from overwhelming her allies
Toot Toot anyone? Goes from mildly useful, mildly annoying fairy to the head of Harry's self-appointed honor guard and in Turn Coat helps Harry take down an Eldritch Abomination by attacking it with a box cutter.
A lot of Harry's friends take a level in Ghost Story. Murphy is now experienced enough with the magical stuff to be as effective in taking them down as Harry was in the firs few books, and she's been taking lessons in combat from Einherjar. And she's doing it without the Sword that has now been identified as Kusanagi. Molly has gotten her illusions to the point where she can work up to six copies of herself almost instantly (something Harry has never been able to do) as well as mentally grapple with the Corpsetaker. Her oldest younger brother Daniel has now become good enough to take on a supernaturally fast sorcerer in a knife fight. Butter's new roommate Bob the Skull has been teaching him about magic, so even if he can't use it, he's damn good with understanding how it works, and therefore screwing with it. And Mortimer Lindquist probably took his long before Changes but... damn. He's one powerful ectomancer.
Ladies and gentlemen, may I present Waldo Butters: Goes from nebbish-y coroner who spent 3 months in a psych ward, to man who wore a polka suit to power a zombie t-rex, ended up as the man who crippled a cult leader while telling him exactly what happened to him.
Oh, and that's just the start. As of Skin Game Butters has become, as Murphy puts it, Batman. Magical Batman. He's started fighting on the front lines with a vest full of cool magical Bob powered gadgets. By the end of the book, he's a knew Knight of the Cross. Armed with a Holy Lightsaber. That's right folks, Waldo Butters goes from a cowardly nerd to a Jedi Knight of the Cross.
Kim Kinnison does this, going in a few chapters of Galactic Patrol from relying almost wholly upon brawn and gadgets to not only taking over multiple enemy officers' minds to achieve his ends undercover, but controlling guard dogs in order to turn off shield generators that are blocking him, and more besides. Lampshaded by Mentor, the Deus ex Machina who grants him his powers, who tells him the advanced training he's getting was inevitable if he survived long enough, and if his mind became mature enough to appreciate the need for it.
Clarissa MacDougall goes from being Sector Chief Nurse to the only female Lensman, justified in canon as her mental abilities have long been evident to the people responsible for her promotion. When she goes back to field work twenty years and five children later, she takes her Level 2 Arisian training (one of only five Lensmen to do this) and becomes truly formidable, controlling several extremely hostile minds simultaneously while piloting her ship, rescuing a prisoner and throwing off the aim of every enemy soldier and pilot who comes close enough to threaten her. And her enemies are all lethal telepaths by nature. It's implied that this is the supreme performance by any Lensman in the field, up until the conclusive battle.
In the Skylark Series, Dorothy Seaton gets a heroic upgrade in the last novel, finally putting her foot down and telling husband and protagonist Richard she'll be fighting alongside him from now, just like her Osnomian woman-friends do alongside their husbands. Though she falls down at the end, all the other humans do as well — the only exception is DuQuesne, whom Dorothy regards as an inhuman monster.
Prince Roger: MacClintock goes from spoiled fop to serious badass when he sees his bodyguard unit sacrifice itself for his sorry behind.
Julie Sims goes from being an energetic high school cheerleader at the beginning of the book to a ruthless, crack shot sniper by the end. (True, as a cheerleader she'd also been training to be an Olympic markswoman, but it's still a noticeable change in attitude, if not in aptitude)
Jeff Higgins goes from a Dungeons & Dragons-playing nerd to being a Badass Biker and officer in one of the New United States elite regiments, in the first book alone tearing into attacking Croat soldiers, taking several of them down before he's wounded by one and saved at the last minute by Captain Gars, aka Gustav II. By 1636, he's a Lt. Colonel Badass, commanding "The Hangman", which is reputed to be the most badass regiment in the USE Army.
Most of the major characters from Percy Jackson and the Olympians go through this, but a special mention for Nico di Angelo, son of Hades. When we first meet him, he's a somewhat nerdy little kid who's really into a collectible card trading game and is ignored in favor of his sister. By the last book, he shows up in a Big Damn Heroes moment, dressed in skull motif armor, radiating an aura of death, wielding a sword of three-foot long Stygian iron, and at the head of an army of the dead, with his father Hades, his stepmother Persephone, and his grandmother Demeter right behind him. Damn.
Percy himself, too, takes a level in the Last Olympian. He bathes himself in the river Styx, after all, and becomes practically invincible and able to fight Titans equally—and BEAT them.
As if the real Roman Republic wasn't Badass enough, John Maddox Roberts' Alternate HistoryHannibal's Children has them take a level or three in reaction to being exiled north of the Alps. When they come back one hundred years later, a Greek thinks that the sound of Roman laughter reminds him of swords clashing against shields. They don't swagger or bully; they're too badass for that. In one battle, an "inexperienced" Roman army under a "second-rate" general faces a veteran mercenary force twice their size and led by Carthage's best general. The Romans are wiped out — but the Carthaginian army is wrecked, with two-thirds of its troops killed outright, and most of the rest badly battered.
In Frederik Pohl's Heechee Saga series, the protagonist Robinette Broadhead (a male, he assures) goes from a Wyoming mold miner to a man who survives an encounter with a black hole then, after his death, saves some children from escaped convicts as a digitally stored personality.
In The Sword of Shannara, Menion Leah goes from ditzy prince who's constantly getting lost to savior at least two cities, defeater of Gríma Wormtongue the mystic Stenmin, and rescuer of two elves, a mighty warrior, and an awesome dwarf. Oh, and he picks up a hot girl along the way.
Tavi, in the first book of Codex Alera: small for his age sheepherder who is handicapped by lacking powers that everyone else has, and survives only because of his quick wits and having (and making) an assorted group of Bad Ass friends. Tavi (sorry, First Lord Octavian) in book 6 of Codex Alera: still wily and intelligent, has an even larger assorted group of badass friends, now a tall, physically powerful experienced warrior and wartime commander who now wields powers practically all his peers can only dream of.
The legion Tavi ends up leading in Cursor's Fury. The beginning of the book makes it clear these are, well, not Alera's best troops. Even the Knights are barely up to snuff, dubbed "Knights Pisces" for their resemblance more of fish flopping around than seasoned warriors. Then, at the end of the book, at the end of a grim, determined battle, they take that appellation and turn it around. Sharks are fish, too, after all...Similarly, the legionnaires are likewise wet-behind-the-ears kids, but by the end the survivors are the Battlecrow Cohort.
In the Dark Heavens trilogy by Kylie Chan, Emma Donahoe goes from an ordinary nanny at the beginning to being able to take down high-level demons at the end. It helps that her employer is the god of martial arts.
In the Dragonlance Chronicles, while the elven princess Laurana had already shown herself to be much smarter, stronger and braver than the Brainless Beauty everyone initially dismissed her as, she finally became a true badass at the Battle of the High Clerist's Tower. At that battle she successfully controlled a Dragon Orb (something that was supposed to be impossible for all but the most powerful wizards to do) and used it to force the attacking dragons into a trap. Then, despite being so exhausted from her use of the Dragon Orb that she could barely even still stand, she rushed to the tower wall and single handedly stared down her Arch-Enemy, the Dragon Highlord Kitiara, to protect the body of her friend, Sturm Brightblade. Laurana's heroism at the High Clerist's Tower led to her being given command of the Whitestone Army where she would prove to be a Four-Star Badass as the Golden General.
In The Demonata, Kirilli Kovacs goes from an incompetent loser to an insane badass over the last few books.
In the Mistborn series, Elend Venture increases massively in competence by the third book. Being Mistborn will do that to a guy... Though one could argue this happened before he became Mistborn — when, without any magic at all, he decided he was tired of being pushed around, and strode into the tent of the commander leading one of the armies sieging his city, stabbed the guy, delivered an ultimatum, then on the way out, killed one of the guy's koloss for information.
Also, Spook, who spends the first two books solidly Overshadowed by Awesome, finally gets a chance to shine in Hero of Ages and show off just the kind of things someone with street savvy and Super Senses can pull off.
Honestly, Vin herself. Our heroine begins the trilogy an emotionally scarred, scrawny teenager with mildly useful but seemingly minor Emotion Bomb-type abilities, and grows from there into the most powerful member of La Résistance, the best assassin in the world, a true Lady of War, an empress, and then, briefly, a goddess. The best part? She does it all without turning into a God-Mode Sue.
Robert A. Heinlein's The Door into Summer features Daniel Boone Davis, an engineer who loses his company to his business partner and secretary/former fiancee only to use time travel to and from the future to gain revenge by starting a competing company using knowledge from the future to drive his former partners out of business, and dropping the dime on his former fiancee's legal and financial indiscretions, and aging up a young girl who had a crush on him to an appropriate age where they could get legally married
Also by Heinlein, in Between Planets the protagonist starts as a naive and sheltered teen but becomes a tough Venusian freedom fighter who probably had killed (off-screen).
Teela Brown spent most of Ringworld getting into trouble through being too useless and naive to know how to stay out of it because she never needed to through being genetically ultra-lucky. When she reappears as a Protector-stage human in the sequel, she is intelligent enough to deduce how to hack into a Puppeteer stepping-disc system and strong enough to fight a full-grown, experienced and rejuvenated Kzin bare-handed and nearly kill him despite deliberately trying as hard as she could to lose.
R.A. Salvatore's Hunter's Blades trilogy features Obould Many-Arrows, who goes from being an Elite Mook leading a Zerg Rush and being manipulated by a group of drow who are causing havok for shits and giggles, to being blessed by a god with super strength, speed, agility, and increased intelligence. He starts bossing around the giants he was partnered with, the drow that were manipulating him, leading his zerg rush with dangerous tactics and foresight, tossing heroes out of the way as if they were nothing, and forcing Drizzt Do'Urden to flee from battle.
He becomes a recognized, if Lawful-Evil aligned, estadist and king who creates a ruling dynasty, set peace treaties and would-be alliances with humans, elves and freaking King Bruenor and his dwarves, and establishing at least a century of orcist peace through the region. That was mainly achieved through asskicking and badassery.
While not in the novels, the 4E source books top it of by having him ascend to a true (demi)godhood upon his death. So he leveled up again.
Inevitably for a military setting, a few characters in Honor Harrington are shown to do this. Most notably, it is one of the main plotlines in book 6 when a technician gets targeted by a much larger bully of a crewman for no reason than being in the wrong place at the wrong time. The ship's authorities know what is going on but the technician is too scared to testify, so strings are pulled and he ends up being invited to spar with a few Marines, which builds up his martial skill, so when the bully confronts him again, he loses.
Being a realistic military setting, he ends up facing disciplinary action for taking the law into his own hands, which Honor sees as his punishment for not testifying in the first place. He gets over it.
On a larger level, this also applies to various star nations and militaries in the setting, with the Graysons going from being a frontier Cult Colony with laughably outdated hardware to one of the setting's premier military powers with the assistance of the Manticorans. A major theme of the Back Story is how the Royal Manticoran Navy made a similar advance, due to their suddenly becoming very important with the discovery of the Manticore Wormhole Terminus, and the resulting need to fend off foreign conquest.
Marta of the 'Dragon Slippers Trilogy'. in the first book, she's just an apprentice dressmaker with a huge romantic streak and happens to be Creels friend. by book two, she's leaping off a flying Dragon's back, landing on another dragon, killing the dragon rider( who had enslaved said dragon) and the leaping onto ANOTHER. all while other dragons/ riders are doing epic battle.
Mac from the Fever Series starts out as an unambitious young woman who loves pretty clothes and partying. By the second book she's a badass evil fae exterminator, and she only gets tougher from there.
Ludovic Leblanc in ‘’City of the Beasts’’ spends most of the novel being annoying, vain, and spouting theories that all turn out to be wrong. But when the expedition's in danger, he jumps into a firefight to save a native child, comes up with a cunning plan, and then manipulates the main bad guy, distracts him, and tries to get him drunk so that they have a chance at escaping. Alex and Nadia actually do most of the day-saving, but it's still pretty awesome.
One of the characters in Richard C. Meredith's We All Died at Breakaway Station is Glenn, Guardian Culhaven, a destroyer captain on a mission into enemy territory to rendezvous with and rescue an admiral who's been scouting the enemy defenses. Unfortunately, he's convinced that feeling fear and being a coward are the same thing. This is his first time facing combat, and Glenn constantly tears himself up about the fact that he's so afraid, (mentally) whimpering ... while he firmly makes decisions that expose him to extreme risk. The narrative makes clear that no one watching him on this mission would dream that he's ever in his life felt fear. He doesn't so much take a level as finally begin to realize that he's a brave man after all.
Reynard himself starts the series as a flashy duelist who's more thief than fighter. This becomes apparent after he duels Isengrim for the first time and gets his ass handed to him without landing a single blow. Years of training with Isengrim slowly transforms him into one of the deadliest duelists on the planet.
Rukenaw is introduced as a peasant girl with no military skill whatsoever. Five years later she's become a captain of men (well, women) who personally crushed an enemy general's skull with her morning star.
Daenerys Targaryen spent most of her early life getting pushed around by Viserys, her crazy older brother. After getting sold off to a warlord, she starts developing self-confidence and wielding her power more effectively. By the end of the first book she's executing the murderer of her husband and using magic to hatch dragons. She later finds herself facing off against sorcerers, commanding armies and ruling city-states.
Rhaegar Targaryen apparently underwent this in the setting's history. He was a scholarly young boy until he found something in an old book that convinced him he needed to become a warrior. He quickly became quite a notable knight.
Lord Manderly goes from having no notable characteristics besides obesity, and generally ignored by the fans, till he shows his true colors, rescues Davos and plots to return Winterfell to the Starks. In the process, he gets revenge on murderers of his son by killing two Freys and serving them in pies at a feast for Roose and Ramsay Bolton. To convince everyone that the pies aren't poison, he eats an enormous helping himself with gusto.
In the first Dunk and Egg short story "The Hedge Knight," Dunk is a Mighty Glacier with very little actual skill at swordplay. By the third book, "The Mystery Knight," he's still a terrible jouster but has fought in a few battles and can now easily dispatch a competent swordsman.
He eventually becomes head of the Kingsguard, the King's own personal bodyguards. Egg? Becomes that same king.
Subverted with Samwell Tarly. After dispatching a White Walker with dragonglass (which was an act of desperation more than anything), his brothers in the Night's Watch start calling him Sam the Slayer, but he insists he's not a badass.
Being the Tom Boy of the Stark sisters, it doesn't take much Genre Savvy to peg Arya as a future Little Miss Badass, but the extent of her badassification can still be surprising. In her first real scene to speak of she receives a sword as a gift, but still relies on her wolf to do her fighting for her until she gets a few lessons from her "dancing master." After a year or two Walking the Earth as an Action Survivor in a war zone, pretty soon she's shanking dudes left and right when the mood strikes her and masterminding prison breakouts in her spare time. After receiving formal assassin training in Braavos and beginning to consciously unlock her latent warg abilities, she's a stone-cold killer at the age of eleven.
Every person who was part of the Italian Resistance took a level in badass in A Thread of Grace. The one who took the most levels had to be Claudia, but that's largely because she started off so low to begin with.
All the kids in The Tomorrow Series. They start off as boringly-normal Australian rural high-schoolers...and end up as some of the most Bad Ass guerrillas in the war, with an international reputation and the enemy mobilizing heavily to try to capture or kill them.
In Veronica Roth's novel Divergent, the entire plot is about how Tris does this.
In The Belgariad, Garion shifts from innocent farmboy to competent swordsman and sorcerer to Godslayer. By the sequel, he's one of the most powerful sorcerors on the planet and someone you absolutely do not want to screw with. His close friend Durnik undergoes a similar change. And oh yeah, speaking of the sequel, remember Sadi and 'Zakath? The drug-addicted eunuch and Empty Shell of an emperor? By the time The Malloreon has ended, 'Zakath has retaken the levels he lost in his time as an Evil Overlord, becoming perhaps the only fencer in the world to fight with a BFS, while Sadi has transformed his love of poisons and mind-altering drugs into a unique style of combat that leaves most of his victims dead or very, very messed-up.
In Dan Abnett's Gaunts Ghosts series, Ludd is diffident and in His Last Command has serious difficulty getting the New Meat to move into action. During Salvation's Reach he goes to stand down three Space Marines, ordering them to call off an attack. And gets them to obey him.
Private Henry from Paul Kidd's trilogy set in the world of Greyhawk. When he first appears in Descent Into The Depths Of The Earth he's somewhat comical in his attempts to light a lantern and being insulted for his sheer incompetence by his sergeant. By the end of the book, he's mowing down Drow by the dozens with his newly acquired magical, self-loading crossbow. By the end of the next book, he's outwitting undead warlords, faerie wizards, and facing down gods without flinching.
In The Last Unicorn, Schmendrick the Magician has spent an indefinite number of years being useless — he has a talent for magic, but no idea how to exercise it, and it only goes off occasionally at random. Then, just in time for the climactic scene, his brain makes a significant connection, he realizes how to say the right words and how to say them differently next time and as many times thereafter as he needs, and for the rest of his career, monsters "worse than afrits surrendered at the mention of his name".
Authorized Peter Pan sequel Peter Pan In Scarlet evolves Slightly, once a cowardly and stupid little brat, into an intuitive and somewhat melancholy figure who fights evil in Neverland armed with a clarinet and the power of rhythm 'n' blues. note (Which, incidentally, is a nice Continuity Nod—in the original play/book, Slightly is said to have a knack for turning tree branches into "whistles".)
Verna in Margaret Atwood's short story "Stone Mattress".
While Septimus Heap is more of a Squishy Wizard in the early books, in Queste he knocks down the rather strong Toll-Man almost on his own.
Bob Howard, the series protagonist, appears to have taken a few levels himself without realizing it. Most of the series takes place from his point of view, and he retains the personality of a snarky I.T. geek in over his head throughout.
The Apocalypse Codex finally lets us see Bob from other characters' perspectives: namely, a very dangerous sorceress and her partner/bodyguard. Both of them have to keep reminding themselves not to take Bob lightly. At the end of the book, it turns out he's so good, the Laundry wants him in Management.
He took a much bigger level in badass in the third book, after he Came Back Wrong. Fortunately for everyone involved it didn't stick.
In the Northland Series, Shan Tasa Aidahan begins as a spoiled, useless load, and then he kills Jakosh Merkana. In fact, this is mostly a fluke, but it inspires him to try harder and learn to actually become a Bad Ass worthy of his names.
When Mundo Cani Dog first appears in The Book of the Dun Cow, he at first seems to be nothing but a self-hating whiner and nuisance. By the final chapter, he has unhesitatingly saved the lives of several turkeys from venomous serpents, personally confronted Cockatrice, and took down the Big Bad through a Heroic Sacrifice, his only weapon a cow's horn.
Deconstructed with Ben. He never becomes an asskicking warrior because he doesn't have the time or equipment to do so, but he does become significantly braver. It's played for laughs where Ben finds a suit of armor that more or less fits him and says he looks like an asskicking warrior. Then he falls over and says he'll leave the fighting to Daniar and Kalak.
Ritchie also becomes braver, and considering he's a tiger in his prime, he should be good in a fight. Zarracka curbstomps him.
Mordak, a formerly mindless weapon, is not only sentient but has psychic powers.
Benji awakens his powers and becomes a formidable warrior.
Taran of The Prydain Chronicles gets this something fierce. He's introduced as a whiny child who is eager to go on an adventure, but has no idea how to fight or find his way through a forest or even swim across a river. When it looks like the prince he was traveling with is dead, he's forced to pull himself together and lead his remaining companions to warn the king about an invading army. In each book, he learns lessons about life, strength, and sacrifice. By the final book, he is a great fighter who the Commont people willingly fight with, he leads an assault against the neighboring evil land, and is able to find the missing magic sword and kill the Big Bad. Gurgi also takes a few levels, going from a rather pathetic creature that begs anyone who comes across him for food to being a strong fighter who, at one point, terrifies the daylights out of two soldiers he battles.
An in-universe example comes up in Stephen King and Peter Straub's The Talisman. On the last leg of his journey, Jack suddenly recalls a scene from the fictional Western Last Train to Hangtown, in which an inept, cowardly character unexpectedly takes on the main villain as payback for killing his brother. The character's cry of "You made a mistake— you shoulda killed both of the Ellis brothers!" resonated so deeply with Jack that he actually uses the line when performing a few of his own moments of badassery.
Poplock in Phoenix Rising, soon after he is introduced. Went from being a frightened small youngster to one so mighty that the Spiritsmith judged he had little to give him that would help in just a few chapters.
Justman takes one level in badass when he saves Superman, Spider-Man and Flash from the bull.
A Mage's Power: Compare Eric at the start, where he freezes in front of monsters, to Eric at midbook, where he kills one by himself. This is all before he's tutored by Dengel, which accelerates his magical education.