Kaori Kanzaki from A Certain Magical Index gradually warms up to Touma after he rescues Index from having her memory reset by Neccessarius. She gets defrosted even further in the novels when Touma helps her get over her past.
Shion in Aikatsu! was very contained, rarely smiling or talking with anyone before meeting Aoi and becoming a member of Powapowa-Puririn. After meeting everyone else she opens up a lot more.
The tsundereishHarem Nanny Miyabi Kagurazaki is tsun-tsun to Kaoru and the rest of the cast at first, but shows more of her dere-dere side when she sees how kind they are to her and Aoi.
Mayu Miyuki is introduced as very much this, but almost immediately becomes a softer character when she meets Kaoru at school, and becomes much deeper and sympathetic when more of her Lonely Rich Kid backstory is revealed.
Iceland is shown to be somewhat of a Deadpan Snarker, but he's described as a possible Sugar-and-Ice Personality in author's notes. Taking in consideration how he interacts with Sealand, it might not be THAT far from canon.
England, as well. He's harsh and bitchy, yes, but if he does care for you, he can be surprisingly tender once in a while.
Germany was frozen for a while as a result of not having any real friends. When he becomes friends with Italy he softens up a lot, although it's not obvious at first glance.
Casca from Berserk. While it wasn't his goal, Guts nonetheless accomplishes this when they got stranded together after a battle (during that time he learns about her backstory and he learns that they weren't so different) and he saved her life twice, turning Casca from a serious, aloof and cold young woman who hardly showed any affection other than for Griffith, to a softer, more sympathetic, and open person who is really just a girl who has a nicer, emotional sidethat she can't show due to her profession. And of course, that hate she had for Guts - turns to love!
Aoki Ko in Bakuman。. when first introduced, she was cold, prone to Brutal Honesty, overly harsh and critical of manga that she did not personally like, refused to listen to any advice her editor might give her, and she did'nt associate with others more than necessary. After a while, through Nakai's determination and later Takagi and Miyoshi's kindness, she take levels in kindness and become a friendlier, more expressive person.
Noelle Silva from Black Clover. She is introduced as extremely proud and unwilling to do anything anyone else says, as she doesn't want to appear weak in front of others. Asta and the gang's willingness to help her gain control of her powers and not look down upon her for her faults get her to warm up to them, but she still maintains her haughtiness around others.
Revy from Black Lagoon fits this trope well. Near the end of the series, we even see her playing with some kids. She was technically teaching them how to shoot people whilst playing, but still, no one actually got shot; which was a first for her.
Bleach: Byakuya Kuchiki, the stereotypical Aloof Big Brother to Rukia who keeps his distance for decades as he wrestles with two conflicting vows (to never break the law, made to his dead parents and to protect Rukia, given to his dead wife, Rukia's sister). This comes to a head in the Soul Society arc when the law decides Rukia must be executed and he's forced to choose which of the two vows he upholds and which he'll break (in keeping with Confucian ideals, he chooses the vow to his parents over the vow to his wife). Once Ichigo resolves the conflict between Byakuya's vows, Byakuya mellows out, reinterprets his two vows in a more flexible way and in general starts becoming more openly warm towards everyone (although he still remains The Stoic on the surface... most of the time). It's still considered shocking when he jokes.
In The Boy and the Beast, Kumatetsu is a man from the Bakemono realm who is a stubborn, brash trained wrestler. He then meets a boy named Kyuta and trains him to be Warrior like him. In the movie, we see how he does care about Kyuta and his efforts to keep him safe.
This is how a young Clare formed her relationship with Teresa of the Faint Smile. She initially opened up for entirely pragmatic reasons, because Clare was determined to stay with Teresa to the point of following her for two days straight, and if she died, Teresa would be implied in a possible murder.
Claire herself may fit the trope as well. She initially closed herself to Raki, but eventually let him know her true feelings for him, and stopping at nothing to protect him.
Ai Shinozaki from Corrector Yui starts as the local Snark Knight, snarky and aloof and really disliking to team work. Slowly, we get to see how she was a Cheerful Child until her dad's death, how her mother's accident affected her, and how Yui's persistence and optimism is slowly thawing her...
Ruki Makino/Rika Nonaka from Digimon Tamers. She even gets a new shirt to make it official. To elaborate: The classic shirt she was pictured with during the entire series showed a broken heart, possibly to emphasize her "rebellious" or "not willing to take sentimental crap" attitude. After she defrosts, her mom buys her a new shirt - the exact same one except with the full heart. Ruki wears it for the rest of the season.
An even longer process for Vegeta in Dragon Ball Z, who started out as a villain and then, after his first semi-HeelFace Turn, a very reluctant, openly disdainful Nominal Hero who would die rather than express any form of geniality or kindness towards his teammates, and very gradually thawed out in a series of baby steps over the course of the next decade, slowly getting around the idea of getting together with Bulma, having a son, voicing remorse for his past actions, expressing love for his family, etc. By the time of Dragon Ball GT (about 25 years after his first appearance), he had spiraled hopelessly into Doting Parent territory for his daughter Bulla, even though he never dropped his gruff exterior.
In some extent, Lucy from Elfen Lied. Most of her arc in the anime. "Defrosting" also includes being slightly less murderous.
Eureka from Eureka Seven. She went from an Emotionless Girl to an average emotional girl thanks to her lover Renton. Story-wise, this means Renton was "The One" for her.
Tokiya Mikagami, the resident ice / water guy. He starts out as a loner brooding over revenge, cares nothing else than that, or Yanagi (who resembles his dead big sister) and is extremely ruthless. However, with further interaction with his friends and the realization that revenge is getting him nowhere and would eventually kill him empty handed, he becomes a warmer individual, while still snarky.
Rasen, a completely pale Emotionless Man who thoroughly believed that he had his emotions erased thanks to his Madogu. Until he realized that he hadn't lost his emotions at all. Then in the epilogue, he could at least talk casually...
Kurei, the supposed 'Big Bad', flip-flops between this. Growing up in the village where he is persecuted just for being 'cursed' (turns out they're wrong) makes him cold, this gets carried over when he is sent to the future, until he meets his soon-to-be stepmother Mori Tsukino, in which he starts warming up... until her husband Mori Kouran noticed his power, takes away his mother, and promptly freezes him up again. Until a girl named Kurenai arrives in his life, gives him the first slap ever, and he starts warming up again... until Kouran blew her up, prompting Kurei to get cold again. He spends the majority of the series being a huge asshole, but after he got his ass kicked by Recca, he slowly starts to show signs that he has been defrosted, made much more obvious in the epilogue.
Satellizer el Bridget from Freezing, who starts off the series as a loner who terrifies everyone at West Genetics due to her tendency to flip out and curb-stomp anyone who sets off her intense haphephobia. Her defrosting process starts soon after she meets Kazuya, and notices that she is not repulsed by physical contact with him.
Sohma Hatori from Fruits Basket. Like the other cursed Sohmas, he was bound to obedience toward Akito, creating a cage-like feeling; as the Dragon of the Zodiac, he was actually one of the ones most affected. When Kana came into his life, she helped to defrost him, and they fell in love. Unfortunately, Akito traumatized Kana to the extent that Hatori had to erase her memory, and he froze his heart more than ever. He defrosted once more upon meeting Tohru (as expected), though to a somewhat lesser extent. It was even lampshaded with references to the snow and the spring that it preceded.
Kyoko Aoi from Future GPX Cyber Formula becomes this as the series progresses. It started showing signs of it when Shinjyo won his first championship, and comes in full-force when she saw Shinjyo and Kaga fighting and in Shinjyo's race in EP 6 of Zero.
Inomata from Gakuen Babysitters slowly learns to loosens up the more she interacts with the Babysitting Club and the Daycare.
Great Teacher Onizuka's Anko Uehara was probably more this before she became what's now considered a Tsundere, since she was violently bitchy to Yoshikawa before the events in Okinawa.
Used in Hyper Police since Poe seems to ignore Tommy's advances for the entire first half of the manga, turning a cold shoulder to him. Lampshaded by the fact that she actually is an elementalist controlling spirits of ice.
A girl in I Wish is referred to as the Ice Princess in her school. Turns out every person she loves end up dying, so she prefers to be cool towards everyone. She does warm up to Lyu-Ui over the course of a brief relationship.
Infinite Stratos: Laura Bodewig is generally referred to as "not so much defrosted as doused in gasoline and lit on fire in the space of a single episode." She is introduced as a Tyke Bomb who hero-worships Ichika Orimura's older sister Chifuyu, and hates Ichika for "weakening" Chifuyu, to the point that she slaps him in the face the moment she meets him. She refuses to associate with anyone else at the school, spending all her time training. When she faces Ichika in the tournament, she does everything in her power to kill him, and when that fails she accidentally activates her Valkyrie Trace (a device she didn't know she had, powered by suppressed emotions). Ichika saves her, and when she wakes up in the hospital, Chifuyu explains that her love for her brother was the reason she drove herself so hard and became someone Laura would admire. The next day, Laura saves Ichika from Rin's rampage, kisses him in front ofthe entire class, and declares him to be her wife. She spends the rest of the series blushing and trying to spend time with him.
Sesshomaru starts out supremely arrogant, concerned only with his own status and power, and willing to kill anyone who gets in his way; by the end of the series he is... still supremely arrogant, but has acquired a Morality Pet and developed into an Anti-Hero with a habit of conveniently just passing through.
And also Kagura, who warms up to Sesshomaru, Inuyasha's group, and especially Kohaku.
Yuriko Star to a certain extent in Irresponsible Captain Tylor. She starts off as being very by the book (since she's in the military) but eventually warms up to the dedicated slacker Tylor (who may or may not be a genius in disguise). It's lampshaded later on when she's transferred and someone under her command is surprised at how polite she is to her subordinates.
In Jin, jerkass and aloof Nokaze gets friendlier and more approachable the longer she spends time with Jin. It helps that she's infatuated with him.
Kaguya from Kaguya-sama: Love Is War used to be an incredibly cold girl (with, in her own words, "a twisted, disfigured heart"), having been raised to only view others as tools to be used. She's already been partially defrosted by the time the series starts due to her having fallen in love with Shirogane, though it does take a while before she completely thaws.
Layla Hamilton seems at first to be an arrogant, ultra-perfectionist bitch who loves ridiculing newcomers at the beginning, but soon we see that she's as hard (if not more) on herself as she is with others and that she loves the Kaleido Stage as much as Sora does. To the point of sacrificing her circus career to gain the Kaleido Stage back from Yuri Killian and foil his revenge on Kalos Eido.
Double Subverted in Kara no Kyoukai. Shiki is an aloof girl with a rather misanthropic nature who doesn't want to relate to anybody. Mikiya tries to get close to her despite her many attempts to keep him away from her and eventually she starts to develop feelings for him. Unfortunately, those feelings are too much for her to handle and she tries to kill him with a knife and then throws herself in front of a car when she realizes she can't bring herself to kill him. But after she recovers from her coma, she starts to accept Mikiya more and he eventually becomes her Living Emotional Crutch.
Shirogane of Karakuri Circus is brusque and even insulting at first, but levels out later on.
Diana Cavendish from Little Witch Academia starts off as a cold, haughtyperfectionist who constantly looks down on Akko's recklessness and lack of work ethic. Over the course of the series she and Akko gain a much better understanding of each other and Akko inspires Diana to be more open with her emotions. She begins smiling more and showing open concern over Akko's well-being, which Sucy Lampshades by commenting that "she's grown soft."
Motoko Aoyama from Love Hina who was probably the hardest shell among the girls Keitaro had to break considering she was always on the defensive. As it turned out she had a good reason. Her attitude stems from her older sister Tsukune whom she worshiped like an idol till she got married. Since then she blames men as a way to cope with the shock.
Chrono is a male example. He used to be (or at the very least, tried to be) just as cold as the ice magic he used, but he mellowed out considerably during the first time skip due to a combination of gaining Fate as a little sister and marrying Amy.
Sheryl Nome of Macross Frontier is the archetypal example. She starts out as the "Galaxy Fairy," an Idol Singer loved by all, tremendously arrogant and haughty. However, her meetings with Alto Saotome (who refuses to take any of her crap) and Ranka Lee (whose innocence and optimism rub off on Sheryl) change her outlook on life, making her a much more sympathetic character. Of course, the series goes one further by giving her a fatal illness and doing everything possible to Break the Haughty, which only makes her even more human.
Princess Arika from Mahou Sensei Negima!. Flashbacks introduce her as a stoic and aloof princess who couldn't stand Nagi, but she eventually fell in love with him. She ended up marrying him after he saved her from her execution.
Natsuki Kuga from Mai-HiME, largely the result of being friends with Shizuru, Mai, and Mikoto, as well as Shizuru's love for her. The ice that she uses as a weapon doesn't defrost, though. Her Mai-Otome incarnation also defrosted and opened up to others after meeting Mai at Garderobe.
Colonel Kati Mannequin is this. She's totally serious, hard nosed military officer who happens to have a Dogged Nice Guy in love with her. She originally uses his crush to positively motivate him to follow orders, but he starts growing on her with his Undying Loyalty, and eventually she cries when she thought he died. In truth he survived, and they got married.
Gundam 00 has Tieria Erde. He's shown as a cold, reserved Deadpan Snarker with not a lot of regard for others, but after his fellow Gundam Meister Neil "Lockon Stratos" Dylandy saves his life, he starts showing more and more emotions and becomes extremely devoted to Lockon. After the four year time skip, he seems to even enjoy having conversations with his and Lockon's partner Setsuna F. Seiei, someone he often used to imply he wanted dead. So it seems he's pretty much grown out of it unless he has to communicate with Neil's brother, Lyle — then he just frosts up. Understandable, to some degree: Lyle is Neil's twin brother who has taken up the "Lockon Stratos" mantle in Celestial Being... four years after Neil's tragic death, whom Tieria might (or not) have fallen in one-sided love with. As the second season advances, Tieria keeps defrosting and exploring his more "human" side...
Then there's Feldt Grace, another Gundam 00 example. She was pretty much the textbook Emotionless Girl (possibly caused by both of her parents dying in a system test of Nadleeh's predecessor unit when she was little) until she started having a crush on Lockon, the defrosting culminating in his death causing the formerly cold and official Feldt to bawl her eyes out while clutching his Haro - the only other person who did that was Tieria. Despite his death, she continued the defrosting and in the second season, she's pretty much a normal if somewhat polite woman.
Nadia from Nadia: The Secret of Blue Water. She initially comes across as a cold, aloof pessimist with no interest in communicating with other people (except animals, notably her pet lion cub, King). She also has a very vicious and somewhat sociopathic temper and stubborn, unbending views about the world. However, she finds herself falling for Jean, and the unlikely relationship that develops between them slowly transforms her, bringing out her caring side as she learns to trust others. There are a few moments when she sometimes lashes out at Jean, but he always forgives her. Likewise, when Nadia isn't brooding over her relationship with Jean, she goes out of her way to do nice things for him (cooking, encouraging him to build a gyrocopter, and even comforting him about his father's death). In fact, they get married at the end. There are at least two moments in the island/Africa arc which are intended to show their friendship turning romantic (a kiss and later, when she admits that Jean is more important to her than the Blue Water in Episode 31). Unfortunately, the writers quickly press reset on their relationship after said "developments" occur in the filler arc. (Episode 35 acts as though most of these events never happened, which is why it is best to disregard them from the continuity of the story.)
Sakura also seems to be apart of this trope since she greatly misunderstood Naruto at first, viewing him as a simply an annoying idiot, but now knows the real Naruto and is warming up to him. She also had a Jerkass Realization early on in the series when even Sasukecriticizes her treatment of Naruto.
Lala Ru from Now and Then, Here and There starts off hating all of humanity after all she's been through; by the end of the story, she has sacrificed herself to keep humanity alive by restoring water to the dead Earth.
Nami. At first, she claims to only want to work with the crew to accomplish her goals, and maintains that for a little while before she eventually betrays them to steal their money. But by that time, she had grown so comfortable with them that she didn't want to leave; even her own sister remarked that it had been a long time since she had seen Nami so emotional.
Nico Robin even more so than Nami. Goes from "betray everyone to survive", a philosophy that she had had for twenty years, to "sacrifice yourself to allow your True Companions to escape", after about a month with the Straw Hats.
Boa Hancock also serves as this, to an extent. She is at first portrayed as a misandrist Ice Queen that clearly lives by the "Screw the Rules, I'm Beautiful!" philosophy, mixing in some Alpha Bitch along with that. Then along comes Luffy, and one Defeat Means Friendship later with her younger sisters, in which he protects them from having their darkest secret revealed, she pours out her heart to him and pretty much immediately afterwards, she falls in love with him.
Echo started out as Vincent's impassiveExtreme Doormat servant. Then, meeting Oz gave her the courage to defy her master and vent about how annoying he is at different intervals, and as more time passed, she eventually developed a Sugar-and-Ice Personality and generally started emoting more often.
Leo would coldly dismiss pretty much anyone who would try talking to him in the middle of reading and was also not very good with children. Meeting Elliot helped him overcome these, to the point that he is now able to call his fellow orphans his siblings. Although it seems like Elliot's death and his own responsibilities as Glen have refrosted him, though in adifferentmanner.
The Pokémon anime's rendition of Mahogany Gym's Pryce. This was because as a young Trainer, he mistakenly thought that his Piloswine had deserted him after a battle they lost, when in reality it was frozen in ice and Pryce never found out. Once Ash and his friends help him resolve the HUGE misunderstanding and he finds his Piloswine again, Pryce cheers up noticeably. His manga rendition, even more so. This time, it's a Lapras that caused the sadness.
A male version with Fakir, as well, though his case is the opposite of Rue's. Most of the first season he's borderline abusive towards his best friend, Mytho, and comes off as incredibly coldhearted and selfish. Around the same time Rue's dark side appears, his nicer side emerges from time to time and we learn why he's been acting like an Ice King through his backstory. By the second season, he's become much nicer and even downright Adorkable at times.
Inner Moka of Rosario + Vampire. She's fairly aware that this has happened to her and greatly dislikes it because, in her opinion, it has a negative effect on her fighting abilities. As of later chapters, she's also become a full blown Tsundere towards the protagonist, Tsukune. By the final chapter, she has completely defrosted into a sweet and cheerful cutie just like the now deceased Outer Moka. Then the epilogue reveals that Outer Moka didn't actually die, but was absorbed into Inner Moka and this had gone completely unnoticed by the characters (including Inner Moka herself) because it overlapped so much with Inner Moka's defrosting.
Oscar de Jarjayes from The Rose of Versailles. She is never mean or snobbish, but she becomes more socially conscious and empathetic as the series progresses — due in part to her friends exposing her to the real world.
Sayo Mutou aka Magdaria from the Rurouni Kenshin anime is an embittered, sadIll Girl who keeps frosting up to anybody who doesn't belong to her Christian circle. In comes Sanosuke Sagara, who forces her to think and feel outside the box and made her rediscover the joy of living. Magdaria still dies, but at least she can go to Heaven with less regrets in her heart.
Rei Hino/Sailor Mars in the Sailor Moon manga was easily this, although her character was a hot blooded and bossy Tsundere in the anime. In the anime, she defrosts specifically for Yuuichiro. Since he lives with her, but only starting in adolescence, it could be considered a late-in-life Childhood Friend Romance, or something.
In Shakugan no Shana, Shana begins the series as an ice queen, cold, not caring for anyone. She sees herself as 'just a Flame Haze' and Yuji as 'just a Torch'. After some heartwarming from Yuji she comes to realize that he is more than just a torch and develops feelings for him when she sees a rival for his affections.
Erina Nakiri in Shokugeki no Soma is a textbook example. She's rather aloof and isolated by default, but when people make an effort to get to know her she actually warms up quickly and turns out to be pretty nice, if a little socially awkward and sheltered. While she starts out as The Rival to Soma and does everything she can to get him expelled when he hurts her pride, over the course of the story she comes to acknowledge his skills as a chef, they end up becoming particularly close after finally making the effort to get to know and understand him (helped by the revelation that he's the son of the man she admires), though she doesn't like for people to notice and they still tend to bicker. But even so she can openly admit that she cares about him and doesn't want him to get expelled.
This is basically the entire dynamic between Limone and Dominura in Simoun, with the former making the latter realize her repressed emotional side and evolve away from a heartless manipulator she is first introduced as. In fact, Limone's Fan Nickname is "Icebreaker" in recognition of this feat.
Tiger & Bunny. Kotetsu T. Kaburagi spends a good deal of time trying to defrost his partner Barnaby Brooks, Jr. - an icy, introverted, Ineffectual Loner. He finally starts making some progress after shielding Barnaby from enemy fire during a fight. But it's worth noting that Barnaby never changes his aloof, distant demeanour towards anyone other than Kotetsu even after being defrosted.
Yui Kotegawa in To Love-Ru. She initially acts bitchy to both Rito and Lala. Over time, she loosens up a bit and even picks on Rito when he has one of his classic "accidents". This ultimately culminates in Chapter 4 of Darkness where she finds herself in Rito's room laying on his bed wearing nothing but a button up shirt confiding to Rito that she finally understands that basically boys will be boys even if they do perverted things. She even implies she wouldn't mind if Rito would like to do "shameless things" with her as long as it's only her. Yui then tries to pass off what she said as a joke and even claims it was all a test afterwards when Rito fumbles for the right response.
Sayaka from Tonari no Kashiwagi-san initially starts cold to everyone who's not Kotone, but eventually warms up to Kazuki while they're dating, and while she tries to hide it, to Yuuto too.
In Umi Monogatari, Kanon starts out aloof, awkward, and unwilling to form connections with people. However, she gradually warms up to Marin.
In Unlimited Fafnir, Lisa Highwalker, who is cold towards Yuu at first, opens up to him slightly in episode 4 after he helps them defeat a dragon. But instead of calling him a family member like she does with the other students, she "upgrades" him to "apprentice student".
In ...Virgin Love, Kaoru doesn't really like people until Daigo's warmer personality starts dethawing him. Even his subordinates notice he's become nicer, increasing his popularity at work.
Yukina from Watashi ni XX Shinasai! is slowly becoming as she experiences love for first time, but it's an extremely slow process.
Imaizumi in Yowamushi Pedal after befriending Onoda. Even his teammates realize how uncharacteristically talkative Imaizumi is when he's with Onoda.
Aki Izayoi of Yu-Gi-Oh! 5D's has her entire appearance in the first season based on this trope. It takes not just one but two duels against Yusei in order to completely thaw her out. (Apparently Defeat Means Friendship only managed to change her from emotionless to tearful the first time round.)
Dr. Atsuko Chiba from Paprika. By the end of the film, she admits her secret crush on Tokita and opens up to him.