Blackadder Goes Forth: Captain Blackadder is not the nicest man in the trench, but he cares enough for his men, specially compared to his ancestors, taking in mind that Edmund in Black Adder the Third killed seven people, whereas this Blackadder only killed a pigeon, although he faced a court martial for it.
Captain Darling, in the final episode, after Melchett sends him to the front line.
Lieutenant George in the same series, when compared to Prince George from the prior one. Prince George was a repulsive, piggish and obnoxious brat with an overactive sex drive, whereas Lieutenant George is more of an naive and Spoiled Sweetover-grown schoolboy who treats the war like a jolly good jape.
That woman you might have seen chilling in prison on another show before breaking out to save a villain? Who's treated by some of the characters as so cool and nice now? Faith. Yeah, that Faith.
Zig-Zagged with Cordelia in Season Three when Xander cheats on her, effectively ousting her from the Gang. Cordelia attempts to return to her old ways, but Harmony has usurped her position and shut the door. Cordelia returns to aid the Scoobies in the Season Three Finale. Her move to Angel cements her growth as a caring person.
Kennedy as of the Season Nine comics. In the final TV season, as well as most of Season Eight, Kennedy was a bitchy, confrontational Jerkass who freely admitted to being a "brat." At the end of Season Eight, Willow dumps her after being Brought Down to Normal, and throughout Season Nine, she's considerably more mellow and kind, particularly around Buffy. As she explains, a significant chunk of her bitchiness revolved around Willow and has mellowed out considerably since the break-up; she even comes right out and says that Willow dumping her was the best thing that ever happened to her.
Deke Shaw from Agents of S.H.I.E.L.D goes from being a pragmatic, usually self-centered survivor of the apocalypse who reluctantly helps the S.H.I.E.L.D. team return to their present time, to taking more of an empathetic role with the team when he accidentally goes back in time himself. In a place where he no longer has to deal with the subjugation of the Kree, he tones down his hard-nosed approach, and even has a few naive, almost childish reactions from the new experiences of living stress-free.
A mix of this and Took a Level in Jerkass with Pierce. In the first season, Pierce was an ass, but his role was mostly as comic relief. In season 2, he becomes an outright villain, but he also becomes more sympathetic and develops more of a Freudian Excuse.
Troy was a bit of a Jerk Jock in Season 1, but in later seasons he's the nicest character on the show.
When he first appeared in the Criminal Minds episode "Profiler, Profiled", Detective Silenski was a racist, condescending Smug Snake who spent the majority of his screen time insulting and belittling the BAU. When he reappears in "Restoration", he's cleaned up his act considerably.
This is how Jeri Hogarth's character arc progresses. In season 1 of Jessica Jones (2015), she's a ruthless corporate lawyer who will stop at nothing to achieve her goals, and is willing to use Kilgrave to get her spouse to agree to lesser terms in the divorce. She realizes the wrongness of her actions when this backfires, resulting in Wendy getting killed, Jeri getting viciously cut all over her face, and Pam getting thrown in jail for killing Wendy in self-defense. By the time season 2 of Daredevil (2015) rolls around, we learn that Jeri hired on Marci Stahl and other Landman & Zack attorneys who escaped indictment when Wilson Fisk was arrested, and in the season 2 finale, she personally approaches Foggy Nelson to offer him a partner position. By Iron Fist (2017), she's a much more cynical person, who turns out to have tended the Rand family's graves for the last 15 years, and helps Danny Rand reclaim his identity upon his return to New York City.
Marci Stahl starts season 1 of Daredevil as a member of Landman & Zack, and both looks and acts like Regina George if Regina George became a lawyer. Foggy manages to break through to her by reminding her that she used to have a soul, and this causes her to turn on her partners, who are complicit in Wilson Fisk's criminal activities. Jeri Hogarth hires her, and when we see Marci in season 2 of Daredevil, she's become a much nicer person.
In Season 1 of Daredevil, Mitchell Ellison is depicted as rather unsympathetic, to mislead the viewers and Ben Urich into thinking that he is Wilson Fisk's mole at the Bulletin. When Karen approaches him in season 2 to get his backing on the Frank Castle story, she's expecting him to see her as a replacement for Ben, and is surprised when Ellison instead proves much more helpful, genuinely shows concern for her and expresses great regret over not having backed Ben's work. A freeze-frame of an assignment board in "Seven Minutes in Heaven" shows that he also has revamped the editorial priorities to focus on important events instead of fluff pieces.
The Doctor. Yeah, you might know him now as a goofy fun-loving guy who likes to save innocents, but his first incarnation wasn't nearly as nice. His very first story featured him kidnapping two humans because he didn't want them exposing his secret, and he often showed no qualms about killing people they encountered. Part of the change was definitely deliberate, as the writers wrote him becoming softer and kinder over his run, but part of the change is due to the change of the show's premise. When the Doctor started out he was a mysterious presence to intrigue the viewers and the show's real protagonists, the companions Ian and Barbara, whereas now much of the Doctor's backstory is known, and he often functions as the main hero.
Note that the Jerkass persona can be chalked up to Early Installment Weirdness: the Doctor is only downright malicious in his first three stories and softens up afterwards, and his negative traits almost entirely disappear by the end of his first series. William Hartnell spends most of Season 2 playing an eccentric loveable grandfather figure who is constantly giggling and calling people "my dear boy". As evidenced in "The Waters of Mars", nasty things happen when the Doctor doesn't have a companion. Maybe the Doctor needed Ian and Barbara's influence to take a level. One Eleventh Doctor comic featuring Ian as companion had the Doctor explicitly say that Ian and Barbara were his influence to become a prosocial force rather than just a selfish old man with a time machine.
It is made rather explicit in the new series that the Doctor needs his companions as a sort of Morality Chain. Amy Pond lampshades this after the Doctor's first instinct is to go for the kill to punish a war criminal. "This is what happens when you're alone for too long."
His twelfth incarnation starts out as a cold, cynical Pragmatic Hero who openly wonders if he is "a good man" anymore. A few seasons worth of Character Development and he's mellowed into a much kinder, if still irascible, man.
Romana decides to spontaneously regenerate to serve as a symbolic change for her doing this. Her old self was smug and a bit of a pain, but when the Doctor's influence on her character made her realise this was not a good way to be, she regenerated into a new body in which she was a lot more amiable and something of the Doctor's Distaff Counterpart.
Lady Mary and Lady Edith in the second series of Downton Abbey. O'Brien similarly becomes much nicer due to residual guilt from the horrible thing she did to Cora.
Arguably Branson - at the beginning of the series he is full of hate and narrow minded, caring more about the revolution in Ireland than anything else and deliberately causing confrontation wherever he went. He seemed to hate the Crawleys simply becuase they are rich. By the end of the series, he is kind and wise , and the one person that everyone goes to for advice or to confide in. He looks close to tears in an episode where his political friend upsets Robert at dinner.
Interesting example as it's heavily implied that this new kindness is caused by him giving up after he crosses the Despair Event Horizon when Sybil dies.
Osha. Compare the Osha that tried kidnapping Bran to the Osha that would willingly give her life for him and Rickon. That's one hell of a difference.
After House Bolton's defeat, Robett Glover states he regrets not helping Jon Snow and House Stark in their greatest hour of need. He asks for forgiveness, and Jon Snow tells him that there is nothing to forgive. Afterwards, he along with other Northerners declare Jon Snow the new King of the North.
Robett: There will be more fights to come, but House Glover will stand behind House Stark...and I will stand behind Jon Snow... *draw his sword* THE KING IN THE NORTH!
Jaime, especially in his interactions with Brienne, and in Season 4, his more introspective and softer demeanour takes people by surprise. Indeed, when he lapses back into his former swagger, as demonstrated by his confrontation with Loras in "The Lion and the Rose", he fails miserably.
When returning in Season 6, Sandor is still pretty cynical but is less of a Jerkass and tries to leave his past behind him to lead a normal life. Even when he returns to being "the Hound" while he's partly motivated by revenge he's also at least partly motivated to avenge the innocents he was living among, pushing him more in an Anti-Hero direction than before.
Daario has certainly mellowed out and acts relatively less hedonistic since being recast. The arrogant bad boy characterization has been toned down considerably.
In Season 5, Stannis is generally mellowing out and becoming more of a people person, whether its praising Sam and encouraging him, being respectful to Jon Snow or taking a more active role in Shireen's life. Then he burns Shireen alive.
Over five seasons, both Blair Waldorf and Chuck Bass on Gossip Girl have gradually become kinder and more accepting. Both are still quite sarcastic and snarky, but they're no longer deliberately malicious or cruel. At least Chuck isn't. Blair followed this trope until somewhere during season four when she turned around and Took a Level in Jerkass instead.
The prime example will have to be Patricia, who was originally a total bitch to Nina (for thinking she hurt Joy somehow, mind you) but, after learning to trust her, became much nicer, despite still being rather tough and sarcastic. Even in other seasons, when she finds new people to hate, she never goes to the same level she did for Nina, showing she did in fact change.
Jerome. Oh, Jerome. At first a manipulative and uncaring, he eventually showed a much better side to Mara when she got to know him, and this put him on the path to becoming a better person. Case in point- in the first season, he allied with Rufus out of jealousy of Sibuna's closeness. In the second season, he started helping Rufus again- to rescue the kidnapped Trudy and find the stolen gem. He took another level in jerkass in season three though.
Ren AKA Kamen Rider Knight was stoic Jerk Ass biker who passed no opportunity to demean Idiot Hero Shinji whenever he could. At the end of the series, he started to care for Shinji so much, he actually cried when Shinji died after being stabbed by a Mirror Monster
Shuuichi Kitaoka AKA Kamen Rider Zolda started as one of the more powerful antagonists. He slowly turned to an Aloof Ally, which later evolved to him having a grudging sort of friendship with the protagonists.
Aikawa Hajime from Kamen Rider Blade started off as an emotionless fighting machine whose only purpose seemed to be defeating the Undead. He acted very aggressively to the other Riders and didn't seem to care if innocent people got caught in the crossfire. His only redeeming quality was his devotion to Amane. As the series progresses, he turns into a much kinder and calmer man who learns to fight to protect, rather than to destroy. This change is attributed to the fact Hajime is actually the Joker Undead, who can take on the form and characteristics of any Undead he defeats. As he defeated and took on the form of the kindhearted Human Undead, he gradually started Becoming the Mask.
Hiiro Kagami started as cold Doctor Jerk with minimal care about patients that was only limited to their physical health and no care for his coworkers whatsover. He warmed up to EmuHojo and they formed Bash Brothers as time passed.
Nico Saiba is one of the reasons for the above example and yet is also an example herself. She was initially as nasty as Taiga if not nastier and settled down in his office to get him to beat Emu up over her petty grudge. Seeing what the Doctor Riders have to fight against and what they fight, she gradually developed into Taiga's surrogate little sister with genuine concern. On the other hand, her WomanChild tendecies remained largely intact so she would still push him around and yell at him for minor things from time to time.
In the early seasons of Little House on the Prairie Nellie's the alpha bitch and Willie's a brat. Both become much nicer as they grow up.
Mordred could also potentially be seen as an example of this. When we last see him in series two he kills two knights (who, to be fair, are trying to kill him) and then smiles before telling Merlin that he will "never forgive and never forget". When we next see him in series five, he's all grown up, saves both Arthur and Merlin, seems to bear no ill will toward Merlin and says things like "The love that binds us is more important than the power we wield."
Ziva David from NCIS has in the past couple of seasons evolved softer qualities including a big sister instinct for McGee. Though it may be those aspects of her personality were always there and she needed to feel at home on the team.
Once Upon a Time puts an interesting spin on this, where characters when introduced are very kind and caring, but subsequent episodes show them to have been cruel and petty in the past.
When introduced Snow White's mother, Queen Eva, seems like an archetypal version of The High Queen. It's then shown as a teenager she was a Rich BitchSpoiled Brat, who took an opportunity to humiliate Cora just because she could. Twice even. This bit her in the ass hard.
Snow White herself was a bratty child once too, with a high sense of superiority. Her mother scolded her for it, and insisted it change. By late Season 3 she freely admits she was a bratty child, which Regina instantly agrees with.
Jefferson, The Mad Hatter, is shown to be friendly man and loving father, who is totally screwed over by Regina. It then turns out that years ago, he quite cheerfully participated in the Break the Cutie and Corrupt the Cutie that Regina was subjected to, which turned her into the Evil Queen. This has the added bonus of her treatment of him come off as rather karmic.
Eventually both Regina and Rumplestiltskin get this, as Love Redeems.
Larry "The Kube" Kubiac on Parker Lewis Can't Lose, who went from a typical bully to a far nicer character that mostly got along with Parker and friends as early as the middle of the first season. One episode even shows that he only plays up the "Bully" role because that's how most people view him, but he's actually a pretty good guy.
The eponymous Sherlock becomes somewhat less of an arsehole between series 2 and 3, especially where Molly Hooper and the Watsons are concerned. This is in contrast to Mycroft, who still remains cynical.
The Klingons went from bad guys to Good Is Not Nice between TOS and TNG. The final TOS movie shows us how. However, it's disputable how much they've changed really when it comes to conquering: they're chummier with the Federation, but they may well be no different to other races.
Daniels later confirms a full reformation of the Klingons part when he tells Archer that they'd become official members of the Federation by the 29th century.
From a specific episode of the original, Elaan from "Elaan of Troyius".
Steve in Stranger Things. He gets redeemed at the end of the first season, and in season two is portrayed as much more mature and kind, even becoming a mentor figure to Dustin.
In Wizards of Waverly Place, the case is arguable for Alex since she grew more evil, but she does grow more responsible and bit more caring too. An example is when she is put into Larytate's Happy Helpers's she finds are a joke and actually turns the club into something that actually is helpful.